Mautrix

* Added option to set default event type class in `EventType.find()`

* Fixed notification disabler breaking when not using double puppeting

* Added `NotificationDisabler` utility class for easily disabling notifications while a bridge backfills messages

* Added support for automatically calling `ensure_registered` if `whoami` says the bridge bot is not registered in `Bridge.wait_for_connection`

* Moved initializing end-to-bridge encryption to before other startup actions

* Added extensible enum class (14)
* Added some asyncpg utilities
* Added basic config validation support to disallow default values
* Added matrix-nio based end-to-bridge encryption helper for bridges
* Added option to use TLS for appservice listener
* Added support for `Authorization` header from homeserver in appservice transaction handler
* Added option to override appservice transaction handling method
* Split `Bridge` initialization class into a more abstract `Program`
* Split config loading