Add config settings for background update parameters (#11980)

changelog.d/11980.misc

docs/sample_config.yaml

synapse/config/_base.pyi

synapse/config/background_updates.py

synapse/config/homeserver.py

synapse/storage/background_updates.py

tests/config/test_background_update.py

tests/rest/admin/test_background_updates.py

tests/storage/test_background_update.py