collab: Restrict usage of the LLM service to accounts older than 30 days (#16133)

This PR restricts usage of the LLM service to accounts older than 30
days.

We now store the GitHub user's `created_at` timestamp to check the
GitHub account age. If this is not set—which it won't be for existing
users—then we use the `created_at` timestamp in the Zed database.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>

crates/collab/src/api.rs

@@ -111,6 +111,7 @@ struct AuthenticatedUserParams {
     github_user_id: Option<i32>,
     github_login: String,
     github_email: Option<String>,
+    github_user_created_at: Option<chrono::DateTime<chrono::Utc>>,
 }
 
 #[derive(Debug, Serialize)]
@@ -131,6 +132,7 @@ async fn get_authenticated_user(
             &params.github_login,
             params.github_user_id,
             params.github_email.as_deref(),
+            params.github_user_created_at,
             initial_channel_id,
         )
         .await?;