Notification (and maybe registrations?) endpoint alias

[fgalan]

A way of using alias for notification endpoints, so instead of using the full URL en the subscription (with host, port and path) the alias will be used. Some benefits of this:

• Subscriptions would be more portable. We don't need to change the subscription if the endpoint changes, only the alias.
• Security could be improved. Some users can have grants to modify the endpoints but not the subscription and the other way around. This needs further analysis, it would need modifications on PEP.

Note that in the case of custom notification, you could manipulate the endpoint to using macros, but the actual content of the endpoints (host, port and path) couldn't be touched.

Alias would be stored in a new collection at MongoDB. A CRUD API has to be designed. Maybe we can use a general collection for configurations, in the case we need some other configurations in the future, beyond endpoint alias.

The main focus is a subscription but the idea can be used also in registrations endpoints.