Remote Join with GraphQL endpoints (aka schema stitching)

[dosco]

What would you like to be added:

Support remote joins with GraphQL endpoints in addition to REST endpoints (currently supported)

Why is this needed:

This would require adding some config options to defined a remote endpoint as a GraphQL one. Any query that includes this remote join would then cause a request to be made to the remote GraphQL endpoint and that nested part of the original GraphQL query be sent over. At this time I'm unsure how variables should be handled.

Majority of the changes would be in the serv/core_remote.go file as it handles most of the current remote join (REST) related code.