Initial GraphQL API implementation

What does this MR do?

Adds the graphql-ruby gem, linking it up to the /api/graphql route.

Are there points in the code the reviewer needs to double check?

We should pay extra-special attention to authentication and authorization.

Why was this MR needed?

%10.0 deliverable

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #34754 (closed)