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/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"
+        );
+    }
 }