[FEATURE] POST subscriptions route

[macolu]

Summary

Add new endpoint to add a member to a list. Fix #118

Not sure this is the way you would have done it. Anyway, I had to implement it for an urgent need, so here it is :-)

Unit test

Integration tests added.

Code style

No code style errors according to phpcs.

Other Information

Code uses methods on Repositories that are not declared with "@method" annotations. I can make another PR on Core to add missing annotations if you want.

[xh3n1]

Could you rebase against master, please @macolu? :)

[macolu]

@xh3n1 done :)

[ramonfincken]

Confirmed working to add a subscriber to a single list.

[ramonfincken]

It has been a year since the start of this PR. What needs to be done to get it into rest-api core ? @macolu @xh3n1

[macolu]

I'm not part of the team, so I can't go any further...

[nfebe]

@michield Can we look into this?

[hurricaneop]

@michield Can we look into this?

you need to clear cache in order to get the new /subscriptions to be recognized