Mautrix

* Added default (empty) value for `unsigned` in the event classes.
* Updated the `PgStateStore` in the client module to fully implement the crypto
`StateStore` abstract class.
* The crypto module now has a `PgCryptoStateStore` that combines the client
`PgStateStore` with the abstract crypto state store.

* **Breaking change (client):** The `whoami` method now returns a dataclass
with `user_id` and `device_id` fields, instead of just returning the
`user_id` as a string.
* Added `delete` method for crypto stores (useful when changing the device ID).
* Added `DECRYPTED` step for message send checkpoints.
* Added proper user agent to bridge state and message send checkpoint requests.

* Improved default event filter in bridges
* The filtering method is now `allow_matrix_event` instead of
`filter_matrix_event` and the return value is reversed.
* Most bridges now don't need to override the method, so the old method isn't
used at all.
* Added support for the stable version of [MSC2778].

* Updated registering appservice ghosts to use `inhibit_login` flag to prevent
lots of unnecessary access tokens from being created.
* If you want to log in as an appservice ghost, you should use [MSC2778]'s
appservice login (e.g. like the [bridge e2ee module does](https://github.com/mautrix/python/blob/v0.11.2/mautrix/bridge/e2ee.py#L178-L182) for example)
* Fixed unnecessary warnings about message send endpoints in some cases where
the endpoint wasn't configured.

* Updated message send checkpoint system to handle all cases where messages are
dropped or consumed by mautrix-python.

* Fixed regression in Python 3.8 support in v0.11.0 due to `asyncio.Queue` type hinting.
* Made the limit of HTTP connections to the homeserver configurable
(thanks to [justinbot] in [64]).

[64]: https://github.com/mautrix/python/pull/64