Feature: Matrix support

[MexHigh]

Summary

I want to be able to use Matrix as a channel.

Motivation / Use case

There are currently no channel options for private and self-hostable messengers. People with strong privacy will not be able to use OpenLobster.

Proposed solution

Implement a Matrix SDK like mautrix-go as a separate channel.

Acceptance criteria

• Clear user-visible behavior described
• Backwards compatibility considered
• Tests or e2e checks defined

Alternatives considered

Not to use Matrix.

Additional context

[lordratner]

Same idea with Nextcloud Talk