This drop the "top level" pipeline aggregators from the aggregation
result tree which should save a little memory and a few serialization
bytes. Perhaps more imporantly, this provides a mechanism by which we
can remove *all* pipelines from the aggregation result tree. This will
save quite a bit of space when pipelines are deep in the tree.
Sadly, doing this isn't simple because of backwards compatibility. Nodes
before 7.7.0 *need* those pipelines. We provide them by setting passing
a `Supplier<PipelineTree>` into the root of the aggregation tree that we
only call if we need to serialize to a version before 7.7.0.
This solution works for cross cluster search because we always reduce
the aggregations in each remote cluster and then forward them back to
the coordinating node. Its quite possible that the coordinating node
needs the pipeline (say it is version 7.1.0) and the gateway node in the
remote cluster doesn't (version 7.7.0). In that case the data nodes
won't send the pipeline aggregations back to the gateway node.
Critically, the gateway node *will* send the pipeline aggregations back
to the coordinating node. This is all managed with that
`Supplier<PipelineTree>`, but *how* it is managed is a bit tricky.
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