Add support for context server extensions (#20250)

This PR adds support for context servers provided by extensions.

To provide a context server from an extension, you need to list the
context servers in your `extension.toml`:

```toml
[context_servers.my-context-server]
```

And then implement the `context_server_command` method to return the
command that will be used to start the context server:

```rs
use zed_extension_api::{self as zed, Command, ContextServerId, Result};

struct ExampleContextServerExtension;

impl zed::Extension for ExampleContextServerExtension {
    fn new() -> Self {
        ExampleContextServerExtension
    }

    fn context_server_command(&mut self, _context_server_id: &ContextServerId) -> Result<Command> {
        Ok(Command {
            command: "node".to_string(),
            args: vec!["/path/to/example-context-server/index.js".to_string()],
            env: Vec::new(),
        })
    }
}

zed::register_extension!(ExampleContextServerExtension);
```

Release Notes:

- N/A

crates/extension_host/src/extension_host.rs

@@ -145,6 +145,14 @@ pub trait ExtensionRegistrationHooks: Send + Sync + 'static {
     ) {
     }
 
+    fn register_context_server(
+        &self,
+        _id: Arc<str>,
+        _extension: WasmExtension,
+        _host: Arc<WasmHost>,
+    ) {
+    }
+
     fn register_docs_provider(
         &self,
         _extension: WasmExtension,
@@ -1267,6 +1275,14 @@ impl ExtensionStore {
                         );
                     }
 
+                    for (id, _context_server_entry) in &manifest.context_servers {
+                        this.registration_hooks.register_context_server(
+                            id.clone(),
+                            wasm_extension.clone(),
+                            this.wasm_host.clone(),
+                        );
+                    }
+
                     for (provider_id, _provider) in &manifest.indexed_docs_providers {
                         this.registration_hooks.register_docs_provider(
                             wasm_extension.clone(),