[FEAT] Add support for sending notifications to a topic (Refactor?)

[yatharthranjan]

Is your feature request related to a problem? Please describe.
Currently, a notification can only be sent to a particular user. Which is sufficient for current use-case of the aRMT app but not future proof.

Describe the solution you'd like
There could be a new endpoint with path /topic/{topic_id} though which it should be possible to send notifications to a particular set of users registered on that topic. A schedule function should be investigated if feasible and necessary at this endpoint. This will need to add a new property to user of type list that will store all it's registrations to various topics. This can also be achieved easily if using FCM as suggested in the alternatives below.

Describe alternatives you've considered
Currently, this is supported by adding a notification with a fcmTopic property. which could be useful if using FCM but will only work with FCM. Also the management of topics is out of the appserver so there is no way of knowing which topics exist, if the message was sent to the correct topic and if the user is on a particular topic or not.

Priority
8

Difficulty
6

Additional context
Will need a new function in the user repo to findByFcmToken

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.