Allow plugins to receive configuration from the nushell configuration (#10955)

# Description

When nushell calls a plugin it now sends a configuration `Value` from
the nushell config under `$env.config.plugins.PLUGIN_SHORT_NAME`. This
allows plugin authors to read configuration provided by plugin users.

The `PLUGIN_SHORT_NAME` must match the registered filename after
`nu_plugin_`. If you register `target/debug/nu_plugin_config` the
`PLUGIN_NAME` will be `config` and the nushell config will loook like:

        $env.config = {
          # ...
          plugins: {
            config: [
              some
              values
            ]
          }
        }

Configuration may also use a closure which allows passing values from
`$env` to a plugin:

        $env.config = {
          # ...
          plugins: {
            config: {||
              $env.some_value
            }
          }
        }

This is a breaking change for the plugin API as the `Plugin::run()`
function now accepts a new configuration argument which is an
`&Option<Value>`. If no configuration was supplied the value is `None`.

Plugins compiled after this change should work with older nushell, and
will behave as if the configuration was not set.

Initially discussed in #10867

# User-Facing Changes

* Plugins can read configuration data stored in `$env.config.plugins`
* The plugin `CallInfo` now includes a `config` entry, existing plugins
will require updates

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

- [ ] Update [Creating a plugin (in
Rust)](https://www.nushell.sh/contributor-book/plugins.html#creating-a-plugin-in-rust)
[source](https://github.com/nushell/nushell.github.io/blob/main/contributor-book/plugins.md)
- [ ] Add "Configuration" section to [Plugins
documentation](https://www.nushell.sh/contributor-book/plugins.html)

crates/nu-plugin/src/lib.rs

@@ -28,6 +28,7 @@
 //!     fn run(
 //!         &mut self,
 //!         name: &str,
+//!         config: &Option<Value>,
 //!         call: &EvaluatedCall,
 //!         input: &Value
 //!     ) -> Result<Value, LabeledError> {

crates/nu-plugin/src/plugin/declaration.rs

@@ -6,6 +6,7 @@ use crate::protocol::{
 };
 use std::path::{Path, PathBuf};
 
+use nu_engine::eval_block;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{ast::Call, PluginSignature, Signature};
 use nu_protocol::{Example, PipelineData, ShellError, Value};
@@ -126,10 +127,48 @@ impl Command for PluginDeclaration {
             value => CallInput::Value(value),
         };
 
+        // Fetch the configuration for a plugin
+        //
+        // The `plugin` must match the registered name of a plugin.  For
+        // `register nu_plugin_example` the plugin config lookup uses `"example"`
+        let config = self
+            .filename
+            .file_stem()
+            .and_then(|file| {
+                file.to_string_lossy()
+                    .clone()
+                    .strip_prefix("nu_plugin_")
+                    .map(|name| {
+                        nu_engine::get_config(engine_state, stack)
+                            .plugins
+                            .get(name)
+                            .cloned()
+                    })
+            })
+            .flatten()
+            .map(|value| {
+                let span = value.span();
+                match value {
+                    Value::Closure { val, .. } => {
+                        let input = PipelineData::Empty;
+
+                        let block = engine_state.get_block(val.block_id).clone();
+                        let mut stack = stack.captures_to_stack(val.captures);
+
+                        match eval_block(engine_state, &mut stack, &block, input, false, false) {
+                            Ok(v) => v.into_value(span),
+                            Err(e) => Value::error(e, call.head),
+                        }
+                    }
+                    _ => value.clone(),
+                }
+            });
+
         let plugin_call = PluginCall::CallInfo(CallInfo {
             name: self.name.clone(),
             call: EvaluatedCall::try_from_call(call, engine_state, stack)?,
             input,
+            config,
         });
 
         let encoding = {

crates/nu-plugin/src/plugin/mod.rs

@@ -217,6 +217,7 @@ pub fn get_signature(
 ///     fn run(
 ///         &mut self,
 ///         name: &str,
+///         config: &Option<Value>,
 ///         call: &EvaluatedCall,
 ///         input: &Value,
 ///     ) -> Result<Value, LabeledError> {
@@ -246,6 +247,7 @@ pub trait Plugin {
     fn run(
         &mut self,
         name: &str,
+        config: &Option<Value>,
         call: &EvaluatedCall,
         input: &Value,
     ) -> Result<Value, LabeledError>;
@@ -264,7 +266,7 @@ pub trait Plugin {
 /// # impl MyPlugin { fn new() -> Self { Self }}
 /// # impl Plugin for MyPlugin {
 /// #     fn signature(&self) -> Vec<PluginSignature> {todo!();}
-/// #     fn run(&mut self, name: &str, call: &EvaluatedCall, input: &Value)
+/// #     fn run(&mut self, name: &str, config: &Option<Value>, call: &EvaluatedCall, input: &Value)
 /// #         -> Result<Value, LabeledError> {todo!();}
 /// # }
 /// fn main() {
@@ -333,7 +335,9 @@ pub fn serve_plugin(plugin: &mut impl Plugin, encoder: impl PluginEncoder) {
                     };
 
                     let value = match input {
-                        Ok(input) => plugin.run(&call_info.name, &call_info.call, &input),
+                        Ok(input) => {
+                            plugin.run(&call_info.name, &call_info.config, &call_info.call, &input)
+                        }
                         Err(err) => Err(err.into()),
                     };
 

crates/nu-plugin/src/protocol/mod.rs

@@ -13,6 +13,7 @@ pub struct CallInfo {
     pub name: String,
     pub call: EvaluatedCall,
     pub input: CallInput,
+    pub config: Option<Value>,
 }
 
 #[derive(Serialize, Deserialize, Debug, PartialEq)]

crates/nu-plugin/src/serializers/json.rs

@@ -106,6 +106,7 @@ mod tests {
             name: name.clone(),
             call: call.clone(),
             input: CallInput::Value(input.clone()),
+            config: None,
         });
 
         let encoder = JsonSerializer {};

crates/nu-plugin/src/serializers/msgpack.rs

@@ -107,6 +107,7 @@ mod tests {
             name: name.clone(),
             call: call.clone(),
             input: CallInput::Value(input.clone()),
+            config: None,
         });
 
         let encoder = MsgPackSerializer {};