Geo: Add PackageFileRegistry to GraphQL

What does this MR do?

Add ability to query PackageFileRegistry with GraphQL, so the Admin UI can use it.

To do

• Feature flag the package file registries field https://docs.gitlab.com/ee/development/api_graphql_styleguide.html#feature-flags
• Adequate spec coverage
• Add to self-service framework doc how to add a new data type and how to add a new blob
• Extract reusable code and tests where reasonable
• Break out a "Add geoNode to GraphQL API" MR => !28454 (merged)
• Break out small MR for the Finder => !30137 (merged)

Closes #199879 (closed)

Screenshots

Request query

{
  geoNode {
    packageFileRegistries {
      nodes {
        id
        packageFileId
        state
        retryCount
        lastSyncFailure
        retryAt
        lastSyncedAt
        createdAt
      }
    }
  }
}

Response body

{
  "data": {
    "geoNode": {
      "packageFileRegistries": {
        "nodes": [
          {
            "id": "gid://gitlab/Geo::PackageFileRegistry/1",
            "packageFileId": "1",
            "state": "synced",
            "retryCount": 0,
            "lastSyncFailure": null,
            "retryAt": null,
            "lastSyncedAt": null,
            "createdAt": "2020-03-09T22:23:52Z"
          }
        ]
      }
    }
  }
}

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

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