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/agent_servers/src/custom.rs

@@ -0,0 +1,59 @@
+use crate::{AgentServerCommand, AgentServerSettings};
+use acp_thread::AgentConnection;
+use anyhow::Result;
+use gpui::{App, Entity, SharedString, Task};
+use project::Project;
+use std::{path::Path, rc::Rc};
+use ui::IconName;
+
+/// A generic agent server implementation for custom user-defined agents
+pub struct CustomAgentServer {
+    name: SharedString,
+    command: AgentServerCommand,
+}
+
+impl CustomAgentServer {
+    pub fn new(name: SharedString, settings: &AgentServerSettings) -> Self {
+        Self {
+            name,
+            command: settings.command.clone(),
+        }
+    }
+}
+
+impl crate::AgentServer for CustomAgentServer {
+    fn name(&self) -> SharedString {
+        self.name.clone()
+    }
+
+    fn logo(&self) -> IconName {
+        IconName::Terminal
+    }
+
+    fn empty_state_headline(&self) -> SharedString {
+        "No conversations yet".into()
+    }
+
+    fn empty_state_message(&self) -> SharedString {
+        format!("Start a conversation with {}", self.name).into()
+    }
+
+    fn connect(
+        &self,
+        root_dir: &Path,
+        _project: &Entity<Project>,
+        cx: &mut App,
+    ) -> Task<Result<Rc<dyn AgentConnection>>> {
+        let server_name = self.name();
+        let command = self.command.clone();
+        let root_dir = root_dir.to_path_buf();
+
+        cx.spawn(async move |mut cx| {
+            crate::acp::connect(server_name, command, &root_dir, &mut cx).await
+        })
+    }
+
+    fn into_any(self: Rc<Self>) -> Rc<dyn std::any::Any> {
+        self
+    }
+}