Friends management API service

Hope this is the right section for my matter, did not find a better one.

I am searching for a provider/ backend service that offers sth like a Friends-List-API.
So a ready to use API that has standard functionalities of friends or contacts-list, like adding friends, handling friendship invitations, returning full friends list, and so on …
My goal is to use it for my Ionic mobile app.
I guess something like that should be available out of the box since its needed almost everywhere. However, I simply cannot find anything appropriate online.

If also other functionalities are included in this services, thats fine.

Any suggestion ? Thanks a lot!