Add basic GraphQL for a Group

What does this MR do?

Adds a very basic GraphQL query for a Group. We add a GroupType and a NamespaceType.

Group attributes :statistics, :custom_attributes, :projects and :shared_projects are not initially supported in order to keep the initial implementation small.

---

Example Query

{
  group(fullPath: "h5bp/first-level-subgroup") {
    id
    webUrl
    name
    path
    description
    visibility
    lfsEnabled
    avatarUrl
    requestAccessEnabled
    fullPath
    fullName
    parent {
      id
      name
      fullPath
    }
  }
}

Results

{
  "data": {
    "group": {
      "id": "59",
      "webUrl": "http://ce.gitlab.dev:5000/groups/h5bp/first-level-subgroup",
      "name": "First Level Subgroup",
      "path": "first-level-subgroup",
      "description": "",
      "visibility": "private",
      "lfsEnabled": true,
      "avatarUrl": null,
      "requestAccessEnabled": false,
      "fullPath": "h5bp/first-level-subgroup",
      "fullName": "H5bp / First Level Subgroup",
      "parent": {
        "id": "8",
        "name": "H5bp",
        "fullPath": "h5bp"
      }
    }
  }
}

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 (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11164)

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

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Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60786