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-protocol/src/config/mod.rs

@@ -73,6 +73,12 @@ pub struct Config {
     pub error_style: ErrorStyle,
     pub use_kitty_protocol: bool,
     pub highlight_resolved_externals: bool,
+    /// Configuration for plugins.
+    ///
+    /// Users can provide configuration for a plugin through this entry.  The entry name must
+    /// match the registered plugin name so `register nu_plugin_example` will be able to place
+    /// its configuration under a `nu_plugin_example` column.
+    pub plugins: HashMap<String, Value>,
 }
 
 impl Default for Config {
@@ -139,6 +145,8 @@ impl Default for Config {
 
             use_kitty_protocol: false,
             highlight_resolved_externals: false,
+
+            plugins: HashMap::new(),
         }
     }
 }
@@ -627,6 +635,22 @@ impl Value {
                     "highlight_resolved_externals" => {
                         process_bool_config(value, &mut errors, &mut config.highlight_resolved_externals);
                     }
+                    "plugins" => {
+                        if let Ok(map) = create_map(value) {
+                            config.plugins = map;
+                        } else {
+                            report_invalid_value("should be a record", span, &mut errors);
+                            // Reconstruct
+                            *value = Value::record(
+                                config
+                                    .explore
+                                    .iter()
+                                    .map(|(k, v)| (k.clone(), v.clone()))
+                                    .collect(),
+                                span,
+                            );
+                        }
+                    }
                     // Menus
                     "menus" => match create_menus(value) {
                         Ok(map) => config.menus = map,

crates/nu-protocol/src/engine/engine_state.rs

@@ -712,6 +712,14 @@ impl EngineState {
         self.config = conf;
     }
 
+    /// Fetch the configuration for a plugin
+    ///
+    /// The `plugin` must match the registered name of a plugin.  For `register nu_plugin_example`
+    /// the plugin name to use will be `"example"`
+    pub fn get_plugin_config(&self, plugin: &str) -> Option<&Value> {
+        self.config.plugins.get(plugin)
+    }
+
     pub fn get_var(&self, var_id: VarId) -> &Variable {
         self.vars
             .get(var_id)
@@ -972,4 +980,27 @@ mod engine_state_tests {
         assert_eq!(variables, vec![varname_with_sigil]);
         Ok(())
     }
+
+    #[test]
+    fn get_plugin_config() {
+        let mut engine_state = EngineState::new();
+
+        assert!(
+            engine_state.get_plugin_config("example").is_none(),
+            "Unexpected plugin configuration"
+        );
+
+        let mut plugins = HashMap::new();
+        plugins.insert("example".into(), Value::string("value", Span::test_data()));
+
+        let mut config = engine_state.get_config().clone();
+        config.plugins = plugins;
+
+        engine_state.set_config(config);
+
+        assert!(
+            engine_state.get_plugin_config("example").is_some(),
+            "Plugin configuration not found"
+        );
+    }
 }