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_example/src/example.rs

@@ -3,6 +3,22 @@ use nu_protocol::{Record, Value};
 pub struct Example;
 
 impl Example {
+    pub fn config(
+        &self,
+        config: &Option<Value>,
+        call: &EvaluatedCall,
+    ) -> Result<Value, LabeledError> {
+        match config {
+            Some(config) => Ok(config.clone()),
+            None => Err(LabeledError {
+                label: "No config sent".into(),
+                msg: "Configuration for this plugin was not found in `$env.config.plugins.example`"
+                    .into(),
+                span: Some(call.head),
+            }),
+        }
+    }
+
     fn print_values(
         &self,
         index: u32,

crates/nu_plugin_example/src/nu/mod.rs

@@ -1,6 +1,6 @@
 use crate::Example;
 use nu_plugin::{EvaluatedCall, LabeledError, Plugin};
-use nu_protocol::{Category, PluginExample, PluginSignature, SyntaxShape, Value};
+use nu_protocol::{Category, PluginExample, PluginSignature, SyntaxShape, Type, Value};
 
 impl Plugin for Example {
     fn signature(&self) -> Vec<PluginSignature> {
@@ -42,12 +42,19 @@ impl Plugin for Example {
                 .named("named", SyntaxShape::String, "named string", Some('n'))
                 .rest("rest", SyntaxShape::String, "rest value string")
                 .category(Category::Experimental),
+            PluginSignature::build("nu-example-config")
+                .usage("Show plugin configuration")
+                .extra_usage("The configuration is set under $env.config.plugins.example")
+                .category(Category::Experimental)
+                .search_terms(vec!["example".into(), "configuration".into()])
+                .input_output_type(Type::Nothing, Type::Table(vec![])),
         ]
     }
 
     fn run(
         &mut self,
         name: &str,
+        config: &Option<Value>,
         call: &EvaluatedCall,
         input: &Value,
     ) -> Result<Value, LabeledError> {
@@ -56,6 +63,7 @@ impl Plugin for Example {
             "nu-example-1" => self.test1(call, input),
             "nu-example-2" => self.test2(call, input),
             "nu-example-3" => self.test3(call, input),
+            "nu-example-config" => self.config(config, call),
             _ => Err(LabeledError {
                 label: "Plugin call with wrong name signature".into(),
                 msg: "the signature used to call the plugin does not match any name in the plugin signature vector".into(),