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/Cargo.toml

@@ -35,6 +35,7 @@ collab_ui.workspace = true
 collections.workspace = true
 command_palette.workspace = true
 command_palette_hooks.workspace = true
+context_servers.workspace = true
 copilot.workspace = true
 db.workspace = true
 diagnostics.workspace = true