assistant2: Add general structure for conversation history (#11516)

This PR adds the general structure for conversation history to the new
assistant.

Right now we have a placeholder button in the assistant panel that will
toggle a picker containing some placeholder saved conversations.

Release Notes:

- N/A

crates/assistant2/src/saved_conversation.rs

@@ -0,0 +1,29 @@
+pub struct SavedConversation {
+    /// The title of the conversation, generated by the Assistant.
+    pub title: String,
+    pub messages: Vec<SavedMessage>,
+}
+
+pub struct SavedMessage {
+    pub text: String,
+}
+
+/// Returns a list of placeholder conversations for mocking the UI.
+///
+/// Once we have real saved conversations to pull from we can use those instead.
+pub fn placeholder_conversations() -> Vec<SavedConversation> {
+    vec![
+        SavedConversation {
+            title: "How to get a list of exported functions in an Erlang module".to_string(),
+            messages: vec![],
+        },
+        SavedConversation {
+            title: "7 wonders of the ancient world".to_string(),
+            messages: vec![],
+        },
+        SavedConversation {
+            title: "Size difference between u8 and a reference to u8 in Rust".to_string(),
+            messages: vec![],
+        },
+    ]
+}