[matrix] notification provider

Just throwing that here, but it would be cool to add a [[matrix]](https://matrix.org/) providers to send notification on a room and/or person (like it is for Telegram)

Compare the existing Telegram notification provider to understand how notification providers are integrated. Then identify the relevant notification entry point and define the Matrix room and/or person behavior; the work is done when Matrix notifications function for the intended recipient types and are covered by the project's existing checks.

Written by the indexing model from the issue text.