Move Aggregator#buildTopLevel() to search worker thread.

[martijnvg]

As part of #98204, aggregation post collection was moved to the search worker thread in the ContextIndexSearcher class.
We should do the same for building top level internal aggregations. The reason for this is that depending on the aggregation implementation that can be an expensive operation and all expensive search related operations should occur on a search worker thread.

The AggregationPhase should just be the place that reduces the top level internal aggregations from the different slices.
An idea is that post collection of the top level aggregations should perhaps also build the internal aggregations? Then AggregationPhase can just use that directly (instead of building it).

[elasticsearchmachine]

Pinging @elastic/es-analytics-geo (Team:Analytics)