Aggregations: Numeric metric aggregations are now formattable

You can now specify `format` in the request definition for most numeric metric aggregations. The exceptions are Percentile_Ranks, Cardinality and Value_Count as the response type of these can be different from the field type so the formatter won't work.

Closes #6812
Yeah, I did think that. The problem is that we don't expose value(String) as a public method on the Java API but each aggregation exposes its own methods (e.g the stats aggregation exposes max(), min(), etc.) I did think about keeping this internal and adding xAsString() methods to each relevant multi value aggregation. Do you think that would be better?