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/zed/src/main.rs

@@ -13,6 +13,7 @@ use clap::{command, Parser};
 use cli::FORCE_CLI_MODE_ENV_VAR_NAME;
 use client::{parse_zed_link, Client, ProxySettings, UserStore};
 use collab_ui::channel_view::ChannelView;
+use context_servers::ContextServerFactoryRegistry;
 use db::kvp::{GLOBAL_KEY_VALUE_STORE, KEY_VALUE_STORE};
 use editor::Editor;
 use env_logger::Builder;
@@ -411,6 +412,7 @@ fn main() {
             IndexedDocsRegistry::global(cx),
             SnippetRegistry::global(cx),
             app_state.languages.clone(),
+            ContextServerFactoryRegistry::global(cx),
             cx,
         );
         extension_host::init(