Move Notification Endpoint service into own package

[GeorgeMac]

In order to support #19783

We need to isolate notification endpoint service definitions into own package (outside kv) and remove URM interactions.
This also works towards our endeavors to better isolate services from one another.

I have an implementation in the pipeline. This issue is for reference for the compute teams records and for organizing timeline.

Success Criteria

• We can rebase the referenced PR and remove need for *kv.Service to implement notification endpoint service.
• Further isolation of tenant responsibilities in the tenant service.