Adds KAS properties to GraphQL

What does this MR do?

Relates to: #322606 (closed)

Adds KAS metadata types to GraphQL API. They'll be accessible through the instance metadata, since KAS is a service part of the GitLab infrastructure.

Here's how I've implemented it.

GraphQL query

query {
  metadata {
    version # don't need to include it necessarily
    revision # don't need to include it necessarily
    kas{
      enabled
      version
      externalUrl
    }
  }
}

GraphQL response

{
  "data": {
    "metadata": {
      "version": "13.11.0-pre",
      "revision": "7fe6ecdfea7",
      "kas": {
        "enabled": true,
        "version": "13.11.1",
        "externalUrl": "/Users/alexand/dev/gitlab-development-kit/gitlab/.gitlab_kas_secret"
      }
    }
  }
}

Considerations

Why did I not add KAS metadata simply as fields of the instance metadata? Instead I created KAS as a sub-type of it.

• I thought is would be more organized this way, since KAS already has 3 readable attributes and I think there could be more in the future. So it will make more sense to have it as its own field, if we decide to make KAS attributes a field of QueryType in the future. I was also afraid to have to create deprecations quickly in the near feature, which is not the end of the world for GraphQL, but it is always an inconvenience.
• I think this looks more natural for the client querying the API.
• I think we might be increasing the query complexity by one or 2 extra points, since with this approach we'll also have to check for a separate policy (InstanceMetadata::KasPolicy), even though I'm applying the same policy (read_instance_metadata) that the MetadataType already requires. Although, this complexity increase seems quite small, specially considering that this endpoint is probably not used in conjunction with very complex queries. There are no joins or too many nested fields that relate to metadata.
• I've decided to make all fields non-nullable with exception of kas.version and kas.externalUrl, since those don't make sense if kas.enabled? == false.

Screenshots (strongly suggested)

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