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/context_servers/src/context_servers.rs

@@ -1,13 +1,16 @@
+pub mod client;
+pub mod manager;
+pub mod protocol;
+mod registry;
+pub mod types;
+
 use command_palette_hooks::CommandPaletteFilter;
 use gpui::{actions, AppContext};
 use settings::Settings;
 
+pub use crate::manager::ContextServer;
 use crate::manager::ContextServerSettings;
-
-pub mod client;
-pub mod manager;
-pub mod protocol;
-pub mod types;
+pub use crate::registry::ContextServerFactoryRegistry;
 
 actions!(context_servers, [Restart]);
 
@@ -16,6 +19,7 @@ pub const CONTEXT_SERVERS_NAMESPACE: &'static str = "context_servers";
 
 pub fn init(cx: &mut AppContext) {
     ContextServerSettings::register(cx);
+    ContextServerFactoryRegistry::default_global(cx);
 
     CommandPaletteFilter::update_global(cx, |filter, _cx| {
         filter.hide_namespace(CONTEXT_SERVERS_NAMESPACE);