Add a way for bots/bridges to be force-joined rather than invited to rooms

In most cases, a bot/bridge will auto-accept invites anyways and when this fails (likely due to any one of the many steps involved failing) it can be confusing for the user. To save on the round trip time, bots/bridges should somehow be able to identify themselves as willing to be force-joined to a room to skip the invite step.

The confusion is largely around that users expect the integration to be added to the room without needing to jump through hoops, so when the integration is stuck as "invited" people often think that the integration is not functioning/responding to their commands when in fact it's just not able to see their commands. By force-joining the integration, the user can more accurately report that something is broken when it doesn't reply.

One approach for this could be a flag in account data (for bots) or in the appservice registration that tells the server what to do during the invite/join sequence. The invite/join sequence over federation would need to allow for the membership to change during the exchange of events, and the auth rules would need to allow this somehow (doubly signed requirement?).

Start by reviewing the appservice registration and account-data approaches, then trace the federation invite/join exchange and its authentication rules described in the issue. Done requires a specified protocol design that permits eligible bots or bridges to be force-joined, including the required membership and signing rules.