assistant2: Add rudimentary chat functionality (#21178)

This PR adds in rudimentary functionality for sending messages to the
LLM in `assistant2`.

<img width="1079" alt="Screenshot 2024-11-25 at 1 49 11 PM"
src="https://github.com/user-attachments/assets/5accb749-c034-4fb2-bf55-3ae5bc9529ad">

Release Notes:

- N/A

crates/assistant2/src/thread.rs

@@ -0,0 +1,23 @@
+use gpui::{ModelContext, Task};
+use language_model::Role;
+
+/// A message in a [`Thread`].
+pub struct Message {
+    pub role: Role,
+    pub text: String,
+}
+
+/// A thread of conversation with the LLM.
+pub struct Thread {
+    pub messages: Vec<Message>,
+    pub pending_completion_tasks: Vec<Task<()>>,
+}
+
+impl Thread {
+    pub fn new(_cx: &mut ModelContext<Self>) -> Self {
+        Self {
+            messages: Vec::new(),
+            pending_completion_tasks: Vec::new(),
+        }
+    }
+}