Updates case.signal_thread_ts after creating the alerts section

[mvilanova]

This pull request includes a change to the create_threaded method in the dispatch_slack plugin to handle the response from the send_message function and update the signal_thread_ts attribute of the case object accordingly. This change ensures the value of signal_thread_ts is the correct one when new alerts (aka signals) are ingested and the message needs to be updated with new counter.

Error handling and response management:

• src/dispatch/plugins/dispatch_slack/plugin.py: Modified the create_threaded method to capture the response from send_message and update case.signal_thread_ts if the response contains a timestamp.