Add create project subscription mutation - GraphQL

What does this MR do and why?

This MR exposes the functionality to create a project subscription through the GraphQL API.

Screenshots or screen recordings

How to set up and validate locally

1. Checkout the branch and start the application
2. Make sure that you are using your EE license
3. Open the GraphQL explorer
4. Ping the mutation

An example query for the mutation would be the following:

mutation {
  createProjectSubscription(input: {
    projectPath: "toolbox/gitlab-smoke-tests",
    upstreamPath: "Commit451/test-3-pub"
  } ) {
    project {
      name
    }
    errors
  }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #425286 (closed)