Replace the synapse.app.appservice and synapse.app.user_dir worker types with more flexible config options

[anoadragon453]

First phase of #10441.

This PR replaces the logic that decides which worker to notify appservices or update the user directory. Originally, you would have to set notify_appservices / update_user_directory to false, as well as setting the worker_type config option for each worker.

Now we follow the same format as run_background_tasks_on, where we have a single config option which both the main process and worker processes read from. If the value matches the worker's current name, then that worker will carry out that task.

TODO:

• Update Complement worker config generator script
• Make a note in UPGRADE.rst - this is currently a backwards-incompatible change
• Update worker documentation

[richvdh]

this looks very similar to #11450... is it still alive?

[dklimpel]

Is this replaced by #12446 and #12654 ?

[anoadragon453]

Oh! Yes, this is old, old, old and can be closed.