assistant2: Add ability to fetch URLs as context (#21988)

This PR adds the ability to fetch URLs as context in Assistant2.

In the picker we use the search area as an input for the user to enter
the URL they wish to fetch:

<img width="1159" alt="Screenshot 2024-12-13 at 2 45 41 PM"
src="https://github.com/user-attachments/assets/b3b20648-2c22-4509-b592-d0291d25b202"
/>

<img width="1159" alt="Screenshot 2024-12-13 at 2 45 47 PM"
src="https://github.com/user-attachments/assets/7e6bab2d-2731-467f-9781-130c6e4ea5cf"
/>

Release Notes:

- N/A

crates/assistant2/src/context.rs

@@ -23,4 +23,5 @@ pub struct Context {
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub enum ContextKind {
     File,
+    FetchedUrl,
 }