Add group_id argument to merge requests resolver

What does this MR do and why?

Add group_id argument to merge requests resolver

Changelog: added

Screenshots or screen recordings

How to set up and validate locally

1. Create some merge requests in a few different projects in the same group
2. Create some merge requests outside of that group
3. Run the following GraphQL query (updating the groupId):

   query {
     currentUser{
       allMergeRequests: authoredMergeRequests {
         count
       }
       myGroupMergeRequests: authoredMergeRequests(groupId: "gid://gitlab/Group/31") {
         count
       }
     }
   }

MR acceptance checklist

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