Distinct avatars per bridged service instance

[HarHarLinks]

Is your feature request related to a problem? Please describe.
I think it's great to have the visualization of an avatar to distinguish things, e.g. using the default octocatbot to bridge a GitLab doesn't represent that properly.
Continuing that thought, this is even more interesting when e.g. your GitLab instance has a custom branded logo you can use.
This is especially important when multiple things are bridged in one room, but improves UX even with singular ones.

Describe the solution you'd like

• Ability to configure avatars either at the config.yml level similar to the default avatar it currently has
• and/or configure avatar at connection level, when creating a connection

Additional context
Related to #178

[Half-Shot]

I'd like to have it so that each service could have it's own distinct user (go-neb) style for this use case, it's just a bit of a refactor job at the moment.

[justinbot]

We ended up getting this with service bots (#573), each bot user can have a different avatar.