[Backend] Quality notifications to multiple BPNs

[ds-crehm]

As user,
I want to be able to send notifications to multiple BPNs,
so that I don't have to create notifications for each BPN individually.

Hints / Details

Implement concept #849

Frontend implementation: #900

Acceptance Criteria

• Input of multiple BPNs is possible, when creating notifications (should be done first as a separate PR -> to unblock frontend story [Frontend] Quality notifications to multiple BPNs #900 )
• During the creation of notifications, a selection of multiple BPNs will lead to the creation of 'duplicate' notifications for each BPN according to the concept
• Notifications can only be sent between two BPNs (sender and receiver)
• A case label is added to the data model
• (if necessary) APIs are adjusted to make filtering/searching for case label possible
• Update User Manual / Admin manual / Arc42
• Update tests

Out of Scope

• Creation of a 'parent' object for 'duplicate' notifications