Add pipeline subscriptions GraphQL resolvers

What does this MR do and why?

Add pipeline subscriptions GraphQL resolvers.

This allows to query the downstream and upstream projects that a project is subscribed to in the GraphQL API

Example query

query getPipelineSubscriptions($fullPath: ID!) {
  project(fullPath: $fullPath) {
    id
    securityDashboardPath
    ciSubscriptionsProjects {
      nodes {
        upstreamProject {
          namespace {
            id
            name
          }
        }
      }
    }
    ciSubscribedProjects {
      nodes {
        downstreamProject {
          namespace {
            id
            name
          }
        }
      }
    }
  }
}

#425284 (closed)

Screenshots or screen recordings

N/A but here's a GraphQL explorer screenshot anyway

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.