Add Fosscord support

[AruMoon]

Describe the solution you'd like
I wanted to have Fosscord support on this bridge. This app has all the same api as Discord, just needed to specify instance endpoint instead of discord.com

[poVoq]

Fosscord is an Discord client. I think you misunderstand what Matterbridge does.

Edit: at least it used to. Well the question would be if they have the same webhook API (slack like).

Edit: According to their Roadmap the webhook support isn't finished yet: https://fosscord.com/roadmap/