[Aggregations] Meta data support

This commit adds the ability to associate a bit of state with each
individual aggregation.

The aggregation response can be hard to stitch back together without
having a reference to the aggregation request. In many cases this is not
available, many json serializer frameworks cache types globally or have a
static deserialisation override mechanism. In these cases making the
original request available, if at all possible, would be a hack.

The old facets returned `_type` which was just enough metadata to know
what the originating facet type in the request was.

This PR takes `_type` one step further by introducing ANY arbitrary meta
data. This could be further <strike>ab</strike>used for instance by
generic/automated aggregations that include UI state (color information,
thresholds, user input states, etc) per aggregation.