Stop using round-tripped PipelineAggregators (backport of #53423)

[nik9000]

This begins to clean up how PipelineAggregators and executed.
Previously, we would create the PipelineAggregators on the data nodes
and embed them in the aggregation tree. When it came time to execute the
pipeline aggregation we'd use the PipelineAggregators that were on the
first shard's results. This is inefficient because:

1. The data node needs to make the PipelineAggregator only to
   serialize it and then throw it away.
2. The coordinating node needs to deserialize all of the
   PipelineAggregators even though it only needs one of them.
3. You end up with many PipelineAggregator instances when you only
   really need one per pipeline.
4. PipelineAggregator needs to implement serialization.

This begins to undo these by building the PipelineAggregators directly
on the coordinating node and using those instead of the
PipelineAggregators in the aggregtion tree. In a follow up change
we'll stop serializing the PipelineAggregators to node versions that
support this behavior. And, one day, we'll be able to remove
PipelineAggregator from the aggregation result tree entirely.

Importantly, this doesn't change how pipeline aggregations are declared
or parsed or requested. They are still part of the AggregationBuilder
tree because that makes sense.