Add Namespace and ProjectStatistics to GraphQL API

What does this MR do?

This introduces a new GraphQL query to get namespace from full_path.

The namespace type can expose the projects, it has a parameter to include or not projects in subgroups.

The project type can expose statistics

query

{
  namespace(fullPath: "work") {
    id
    projects(includeSubgroups: true, first: 3, after: "MjU=") {
      edges {
        cursor
        node {
          id
          fullPath
          statistics {
            commitCount
            storageSize
            repositorySize
            lfsObjectsSize
            buildArtifactsSize
            packagesSize
          }
        }
      }
    }
  }
}

result

{
  "data": {
    "namespace": {
      "id": "60",
      "projects": {
        "edges": [
          {
            "cursor": "MjQ=",
            "node": {
              "id": "24",
              "fullPath": "work/TestPage",
              "statistics": {
                "commitCount": 16,
                "storageSize": 337146,
                "repositorySize": 335544,
                "lfsObjectsSize": 0,
                "buildArtifactsSize": 1602,
                "packagesSize": 0
              }
            }
          },
          {
            "cursor": "MjI=",
            "node": {
              "id": "22",
              "fullPath": "work/minimal-ruby-app",
              "statistics": {
                "commitCount": 22,
                "storageSize": 107102,
                "repositorySize": 94371,
                "lfsObjectsSize": 0,
                "buildArtifactsSize": 12731,
                "packagesSize": 0
              }
            }
          },
          {
            "cursor": "MjM=",
            "node": {
              "id": "23",
              "fullPath": "work/test-pages",
              "statistics": {
                "commitCount": 17,
                "storageSize": 123450,
                "repositorySize": 115343,
                "lfsObjectsSize": 0,
                "buildArtifactsSize": 8107,
                "packagesSize": 0
              }
            }
          }
        ]
      }
    }
  }
}

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team