Add query inside aggregations

[To-om]

The aim of this feature is to add _query field in selectable aggregations (avg, min, max ,sum and count).
With this feature, it could be interesting to have several aggregations of the same type, with different filter. To prevent name collision, an optional name can be set to any aggregation.
For example:

{
  "query": { "_field": "status", "_value": "new" },
  "stats": [{
    "_agg": "time",
    "_fields": ["createdAt"],
    "_interval": "1h",
    "_name": "documentsOverTime",
    "_select": [{
        "_agg": "count",
        "_name": "LowPriorityCount",
        "_query": { "_field": "priority", "_value": "low" }
      },
      {
        "_agg": "count",
        "_name": "highPriorityCount",
        "_query": { "_field": "priority", "_value": "high" }
      }
    ]
  }]
}