[MatrixRTC] Add extensible event fallback for call notifications.

[Half-Shot]

This allows unsupporting clients to be aware when a call is started, and additionally allows passing in a guest URL when the caller's infrastructure provides one.

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• Sign-off given on the changes (see CONTRIBUTING.md).