Now that the function to `INSERT` and `DELETE` from MariaDB is set up, this diff handles setting up the endpoint, responder and validator, as well as the action method that'll be called via
`useServerCall` client-side.
To not block review while I figure out how to alert the client of a message pin, I've created a follow-up task to decide what to replace Promise<void> with: https://linear.app/comm/issue/ENG-3392/alert-the-client-of-a-message-pin-unpin
Linear: https://linear.app/comm/issue/ENG-3187/set-up-responders-endpoints-and-related-functions-to-toggle-a-message
Depends on D6929