Fix create/update timestamp issues in GraphQL and REST

[etiennedi]

Problem 1

When updating an object (using PUT /v1/objects/{id}) we lose creationTimeUnix. It seems to be missing somewhere around here.

Problem 2

Both fields creationTimeUnix and lastUpdateTimeUnix can only be retrieved using the REST API, but aren't accessible through GraphQL

Goals

• Fix the issue of losing the creation timestamp during an update and prevent regression by adding new test(s)
• Expose both fields in GraphQL Get as part of _additional { } props alongside vector and id`. E.g. the following query should be possible:

     {
    Get {
      Article {
        text
        _additional {
          vector
          id
          creationTimeUnix
          lastUpdateTimeUnix
        }
      }
    }   
  }

• If required, adjust documentation

Note

GraphQL (due to its history and relations to Javascript/NodeJS) does not support int64 numbers. Therefore we cannot display a true unix timestamp. The best solution would probably be to convert the timestamp to a string and make the two new fields string types in the GraphQL API