Allow Matrix Puppeting from other homeservers (or clarify error status if not possible)

[ShadowJonathan]

(Almost verbatim copy of mautrix/telegram#520)

I noticed this while looking through the code for puppeting, it seems that the puppet is trying to send the access token to the same server the application service is currently connected to, is it (simply) possible for the puppeting mechanic to recognise when the access token is not pointing to the currently-connected server, and readjust accordingly?

Or is puppeting through an access token a feature which is granted from the status of Application Service? Does that mean that puppeting through other matrix homeservers will never work? If so, can the error message of "Failed to verify access token." be changed to "Cannot puppet users from another homeserver" when users attempt to give bridges their access token?

[ShadowJonathan]

This error seems to happen when switch_mxid is invoked, after which it'll self.start() the puppet, and try to self.intent.whoami() without considering or switching the server to connect to.

This raises InvalidAccessToken(), as the access token for another server isn't valid on the homeserver the bot is currently connected to.