Subscriptions support

Hello all,

I created this request as the community is desperately awaiting subscriptions in Apollo federation. I did this for gathering information, as it is obvious that subscriptions can't be implemented quickly because there seem to be non trivial problems on the way. The intent is to gather high level information around:

- how subscriptions work in general

- how they could be implemented for federation

- what the problems are on the way

- what could be done to solve those problem

Please everybody to contribute as it seems demanded and I guess a lot of people are willing and able to help!

Thank you all!

Regards,

Christian

Start by reviewing the issue's 16-comment discussion and the requested areas: general subscription behavior, federation implementation, obstacles, and possible solutions. No files, tests, or concrete implementation entry points are named, and the issue does not define a specific completion condition.

Written by the indexing model from the issue text.