Add Mastodon support

[git-sid]

Is your feature request related to a problem? Please describe.
Matterbridge doesn't support Mastodon.

Describe the solution you'd like
Have Mastodon as a supported messaging channel.

Describe alternatives you've considered
Letting the community add the Mastodon support. However, this seems rather difficult without documentation, see #657

[poVoq]

Relevant ActivityPub Go library:
https://github.com/go-fed/activity

But IMHO the protocol doesn't lend itself very well to real-time chat. Might be still cool though.

[Helcaraxan]

Late to the party but am doing some long overdue issue management.

I'd agree with @poVoq on Mastodon (despite it being very cool) not being well-suited to Matterbridge.

@git-sid would you be able to elaborate on how you would see this integration working in relationship to the other real-time chat protocols?