GraphQL: Investigate proxying GraphiQL from gitlab.com

Dependent on #2993

We should investigate what it would take to access customers-dot GraphiQL from a Gitlab.com proxy.

Problem

This would mainly help to communicate exactly what is available through GL.com over what is available in CustomerDot. The differences might be small, but it helps to build the right query.

Proposal

• Add remote schemas of the CustomersDot types: https://www.graphql-tools.com/docs/remote-schemas/.

Result

Next steps (if any)

How will we measure success?