Show bridged contacts on the contact list

When a Matrix user wants to start a conversation with a bridged contact (i.e. a contact on other platforms like Telegram or Whatsapp), the easiest way is to use that platform’s native client because it usually has a contact list.

Doing it from a Matrix client is much more unintuitive and requires jumping through a series of hoops; the user needs to open a DM with the bridge bot and then use written commands to find that contact and start a conversation.

This makes users less likely to use a Matrix client as a replacement for their native Telegram or Whatsapp clients, and therefore, it makes them less likely to start regular Matrix use. Letting the Matrix client show a list of bridged contacts would solve this problem.

To this end, the bridge should access the list of contacts on the other platform; create a puppet for each of those bridged contacts and add all those puppets to a community. This alone would allow the user to see a complete list of all her Telegram contacts and easily start a conversation with any of them, from any of her Matrix clients, just by opening that community. An issue has been created for mautrix-telegram here:

https://github.com/tulir/mautrix-telegram/issues/499

Additionally, Matrix clients which show a local contact list should also show a list of all the members in the communities that the user is a part of. That way, every bridged contact would appear on that list and it would be easy to visually find them, and start a conversation with any of them. That isn't up to the bridge, though, so another issue will be created for that.