Expose committers on mergeRequest GraphQL type

What does this MR do and why?

With #347325 (closed), we need to be able to retrieve a list of committers to a particular MR within GraphQL.

This MR exposes that value and applies a complexity of 5 to the field.

How to set up and validate locally

1. Retrieve a list of MR's using GraphQL using your preferred interface. If you don't have one, you can use [GDK_HOST]/-/graphql-explorer
2. Check that the committers are shown for each MR listed

An example query to run:

query {
  group(fullPath: "gitlab-org") {
    mergeRequests {
      nodes {
      	id
      	title
      	committers {
          nodes {
            id
            name
          }
        }
      }
    }
  }
}

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 #352029 (closed)