Add requires_zed_cla column to channels table

Don't allow granting guests write access in a call where the channel
or one of its ancestors requires the zed CLA, until that guest has
signed the Zed CLA.

Co-authored-by: Marshall <marshall@zed.dev>

crates/collab/src/api.rs

@@ -69,7 +69,7 @@ pub async fn validate_api_token<B>(req: Request<B>, next: Next<B>) -> impl IntoR
 
 #[derive(Debug, Deserialize)]
 struct AuthenticatedUserParams {
-    github_user_id: i32,
+    github_user_id: Option<i32>,
     github_login: String,
     github_email: Option<String>,
 }