Matrix Notification Transport

### Is your feature request related to a problem?

no

### Describe the solution you'd like

Hello,

I wonder if it would be possible to implement a new **notification transport method** using **Matrix**, so notifications are sent to users via the matrix.org protocol, instead of Email or Webhook, which are already in place.

Maybe the settings could allow specifying a matrix account id `@authentik:server.tld` and an "application password", so users get Authentik notifications sent from this account to their matrix user.

I don't know the internals or architecture of Authentik enough yet, maybe this is also not an enviable feature.

Regards

### Describe alternatives that you've considered

no-change

### Additional context

_No response_

The issue names existing Email and Webhook notification transports but no files or tests. Start by locating those transport implementations and their configuration entry points, then determine how a Matrix account and application password would fit the notification architecture. Done means a supported Matrix transport can be configured and sends Authentik notifications to Matrix users.

Written by the indexing model from the issue text.