assistant2: Sketch in sending file context to model (#21829)

This PR sketches in support for sending file context attached to a
message to the model.

Right now the context is just mocked.

<img width="1159" alt="Screenshot 2024-12-10 at 4 18 41 PM"
src="https://github.com/user-attachments/assets/3ee4e86a-7893-42dc-98f9-982aa202d310">

<img width="1159" alt="Screenshot 2024-12-10 at 4 18 53 PM"
src="https://github.com/user-attachments/assets/8a3c2dd7-a466-4dbf-83ec-1c7d969c1a4b">

Release Notes:

- N/A

crates/assistant2/src/context.rs

@@ -0,0 +1,14 @@
+use gpui::SharedString;
+
+/// Some context attached to a message in a thread.
+#[derive(Debug, Clone)]
+pub struct Context {
+    pub name: SharedString,
+    pub kind: ContextKind,
+    pub text: SharedString,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub enum ContextKind {
+    File,
+}