Make background updates controllable via a plugin (#11306)

changelog.d/11306.feature

docs/modules/background_update_controller_callbacks.md

docs/modules/writing_a_module.md

setup.py

synapse/module_api/__init__.py

synapse/storage/background_updates.py

tests/push/test_email.py

tests/rest/admin/test_background_updates.py

tests/storage/test_background_update.py

tests/storage/test_event_chain.py

tests/storage/test_user_directory.py

tests/unittest.py