Send some ephemeral events to appservices (#8437)

changelog.d/8437.feature

mypy.ini

synapse/appservice/__init__.py

synapse/appservice/api.py

synapse/appservice/scheduler.py

synapse/config/appservice.py

synapse/handlers/appservice.py

synapse/handlers/receipts.py

synapse/handlers/sync.py

synapse/handlers/typing.py

synapse/notifier.py

synapse/storage/databases/main/appservice.py

synapse/storage/databases/main/receipts.py

...age/databases/main/schema/delta/59/19as_device_stream.sql

tests/appservice/test_scheduler.py

tests/storage/test_appservice.py