POC to retrieve a list of open MRs that change a specific file

What does this MR do and why?

It is a proof of concept for a GraphQL endpoint which lists open MRs which alter a given file. The query will need some work as it is taking 111.411 ms to run on a large repository for the master branch README.md file. See Postgres.ai query plan.

The GraphQL query below is running in GDK on a new project with a 3 merge requests targeting the 'main' branch and another merge request targeting a 'new-branch'.

GraphQL query:

query getOpenMrsForBlob {
    project(fullPath: "root/blob-mr-test"){
      name
      repository{
        new_branch_head: blobs(ref: "d732ea9f4bab32d7e6bdf37bca609c30ad44ef29", paths: ["README.md", "newfile", "newfile1.md", "newfile2.md", "newfile3.md"]){
          nodes{
            name
            openMrsForBlob(targetBranch: "new-branch"){
              title
              webPath
            }
          }
        }
        default_main_head: blobs(paths: ["README.md", "newfile", "newfile1.md", "newfile2.md", "newfile3.md"]){
          nodes{
            name
            openMrsForBlob(targetBranch: "main"){
              title
              webPath
            }
          }
        }
      }
    }
  }

Returns:

{
  "data": {
    "project": {
      "name": "blob mr test",
      "repository": {
        "new_branch_head": {
          "nodes": [
            {
              "name": "README.md",
              "openMrsForBlob": [
                {
                  "title": "creates new file, changes readme, targets new branch",
                  "webPath": "/root/blob-mr-test/-/merge_requests/2"
                }
              ]
            },
            {
              "name": "newfile",
              "openMrsForBlob": [
                {
                  "title": "creates new file, changes readme, targets new branch",
                  "webPath": "/root/blob-mr-test/-/merge_requests/2"
                }
              ]
            }
          ]
        },
        "default_main_head": {
          "nodes": [
            {
              "name": "README.md",
              "openMrsForBlob": [
                {
                  "title": "changes readme",
                  "webPath": "/root/blob-mr-test/-/merge_requests/1"
                }
              ]
            },
            {
              "name": "newfile1.md",
              "openMrsForBlob": [
                {
                  "title": "changes newfile1 newfile2",
                  "webPath": "/root/blob-mr-test/-/merge_requests/3"
                },
                {
                  "title": "Changes all 'new files'",
                  "webPath": "/root/blob-mr-test/-/merge_requests/4"
                }
              ]
            },
            {
              "name": "newfile2.md",
              "openMrsForBlob": [
                {
                  "title": "changes newfile1 newfile2",
                  "webPath": "/root/blob-mr-test/-/merge_requests/3"
                },
                {
                  "title": "Changes all 'new files'",
                  "webPath": "/root/blob-mr-test/-/merge_requests/4"
                }
              ]
            },
            {
              "name": "newfile3.md",
              "openMrsForBlob": [
                {
                  "title": "Changes all 'new files'",
                  "webPath": "/root/blob-mr-test/-/merge_requests/4"
                }
              ]
            }
          ]
        }
      }
    }
  }
}

MR acceptance checklist

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How to set up and validate locally

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

Related to #469574 (closed)