assistant_tools: Add fetch tool (#26999)

This PR adds a new `fetch` tool to the set of tools the Assistant has
available.

This tool accepts a URL and fetches the content as Markdown.

<img width="1394" alt="Screenshot 2025-03-18 at 11 52 21 AM"
src="https://github.com/user-attachments/assets/e5bcde14-a0dd-4835-9d42-8f45def68f4d"
/>

<img width="1394" alt="Screenshot 2025-03-18 at 11 52 37 AM"
src="https://github.com/user-attachments/assets/3bcce4f5-f61b-40d7-8b30-2c673ce3c06a"
/>

Release Notes:

- N/A

crates/zed/src/main.rs

@@ -472,7 +472,7 @@ fn main() {
             prompt_builder.clone(),
             cx,
         );
-        assistant_tools::init(cx);
+        assistant_tools::init(app_state.client.http_client(), cx);
         repl::init(app_state.fs.clone(), cx);
         extension_host::init(
             extension_host_proxy,