Add option to query count for 'countable' tables field to 'default' gql query

[connoratrug]

Is your feature request related to a problem? Please describe.
When fetching relational/graph data to display is there may be the need to show the number of items for a 'countable' field. For example when viewing the list of variables it might be relevant to show the number of repeats.

Describe the solution you'd like

  Variables({
    name
    repeats {
       _count
    }
  }

should return
[{ name: 'foo', repeats: { _count: 3} }]

only countable field should be allowed to contain a '_count ' if a '_count is added no other ( non agg ) fields can be requested

Describe alternatives you've considered

• Fetching all rows and counting the result, down side; slow and in case of large sets max length might be research
• Other query layouts may be possible

Additional context
Move towards optional stronger query syntax ( filter and agg at the field level) instead of different query types ( _agg, groups_by, ..)