Bring optional dedicated relay bot to mautrix-signal

[maltee1]

mautrix-telegram uses a dedicated relay bot, which improves the experience. While signal itself does not allow creating bots the way telegram does, some people might have a spare phone number to use for such purposes. There probably needs to be a different process for logging in, to allow for captcha. Of course, the current method of relaybotting through one of the users must stay, because not everyone has a spare phone number.

[tulir]

As mentioned on https://docs.mau.fi/bridges/general/relay-mode.html, the correct way is making a separate account for the relaybot. A separate system for dedicated bots will not be added, but some option to designate a mxid as the primary relay user for a bridge and allowing normal users to run specific commands as that user would be fine (mostly set-relay and bridge if that becomes a thing). Could also do the set-relay part implicitly somehow.

[maltee1]

Thanks, that part of the documentation has escaped me somehow. I think not having to run set-relay from the relaybot account would be a must, because that would require logging into that account and make it impossible to use for everyone who doesn't have the password. An implicit set-relay makes sense, because the only reason for having a relaybot in the chat would be to relay messages.

[maltee1]

fixed in https://github.com/mautrix/signal/pull/293