Feature Idea: Subscription contact groups

[XavierChanth]

Is your feature request related to a problem? Please describe.

This originates from the idea that someone could host their newsletter on atmosphere pro.
The problem is that the newsletter issuer would have to manually update their contact group for the newsletter.

Describe the solution you'd like
Be able to create a new type of contact group, where other people could subscribe/request to be added to someone's contact group.

Describe alternatives you've considered
None so far.

Additional context
Please ask any questions if I did not explain this well, and I will update the card.

[gkc]

See also atsign-foundation/at_client_sdk#380

[ksanty]

@XavierChanth I moved this up to med priority since the above issue is med.