Feature Request: Set ghost user avatars

[wasabipesto]

I would appreciate a way to set bridged IRC user's avatars without having to use the synapse admin API. It will probably need to be an owner-only command in network rooms.

Example commands, from mautrix-signal:
set-avatar <mxc:// uri> [mxid] - Set an avatar for a ghost user.
remove-avatar [mxid] - Remove the avatar for a ghost user.