Implement knocking on rooms

*This issue was originally created by [**@S7evinK**](https://github.com/S7evinK) at .*

Version 1.1 of the spec introduced knocking on rooms (MSC2403). We currently don't implement the following endpoints:

Client API:
`POST /_matrix/client/v3/knock/{roomIdOrAlias}` [spec](https://spec.matrix.org/latest/client-server-api/#post_matrixclientv3knockroomidoralias)

[further reading](https://spec.matrix.org/latest/client-server-api/#knocking-on-rooms)

Server-Server API:
`GET /_matrix/federation/v1/make_knock/{roomId}/{userId}` [spec](https://spec.matrix.org/latest/server-server-api/#get_matrixfederationv1make_knockroomiduserid)
`PUT /_matrix/federation/v1/send_knock/{roomId}/{eventId}` [spec](https://spec.matrix.org/latest/server-server-api/#put_matrixfederationv1send_knockroomideventid)

[further reading](https://spec.matrix.org/latest/server-server-api/#restricted-rooms)