acp: Support launching custom agent servers (#36805)

It's enough to add this to your settings:

```json
{
    "agent_servers": {
        "Name Of Your Agent": {
            "command": "/path/to/custom/agent",
            "args": ["arguments", "that", "you", "want"],
        }
    }
}
```

Release Notes:

- N/A

crates/language_model/src/language_model.rs

@@ -643,11 +643,11 @@ pub trait LanguageModelProvider: 'static {
     fn reset_credentials(&self, cx: &mut App) -> Task<Result<()>>;
 }
 
-#[derive(Default, Clone, Copy)]
+#[derive(Default, Clone)]
 pub enum ConfigurationViewTargetAgent {
     #[default]
     ZedAgent,
-    Other(&'static str),
+    Other(SharedString),
 }
 
 #[derive(PartialEq, Eq)]