Backend: Endpoint for prefill inputs

Summary

As a part of Add support for spec:inputs in GitLab CI pipelines (&16321 - closed); create an endpoint to fetch prefill inputs from the CI config.

• Required issue: Backend: Fetch spec from CI Config (#519943 - closed)
• Unlike Ci::ListConfigVariablesService, we don't have to use ReactiveCaching.

Create a GraphQL endpoint to return the spec data of the project CI config.