flow: Add concrete types for server stream events
flow: Add concrete types for server stream events

These actions are created when receiving a stream server event about:
 * create
 * update
 * delete
 * occupy

We do handle the first 3 correctly and do not process the last one yet.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile