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/db/tables/channel.rs

@@ -9,6 +9,7 @@ pub struct Model {
     pub name: String,
     pub visibility: ChannelVisibility,
     pub parent_path: String,
+    pub requires_zed_cla: bool,
 }
 
 impl Model {