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/src/extension_manifest.rs

@@ -75,6 +75,8 @@ pub struct ExtensionManifest {
     #[serde(default)]
     pub language_servers: BTreeMap<LanguageServerName, LanguageServerManifestEntry>,
     #[serde(default)]
+    pub context_servers: BTreeMap<Arc<str>, ContextServerManifestEntry>,
+    #[serde(default)]
     pub slash_commands: BTreeMap<Arc<str>, SlashCommandManifestEntry>,
     #[serde(default)]
     pub indexed_docs_providers: BTreeMap<Arc<str>, IndexedDocsProviderEntry>,
@@ -134,6 +136,9 @@ impl LanguageServerManifestEntry {
     }
 }
 
+#[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]
+pub struct ContextServerManifestEntry {}
+
 #[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]
 pub struct SlashCommandManifestEntry {
     pub description: String,
@@ -205,6 +210,7 @@ fn manifest_from_old_manifest(
             .map(|grammar_name| (grammar_name, Default::default()))
             .collect(),
         language_servers: Default::default(),
+        context_servers: BTreeMap::default(),
         slash_commands: BTreeMap::default(),
         indexed_docs_providers: BTreeMap::default(),
         snippets: None,