This adjusts the `buckets_path` parser so that pipeline aggs can
select specific buckets (via their bucket keys) instead of fetching
the entire set of buckets. This is useful for bucket_script in
particular, which might want specific buckets for calculations.
It's possible to workaround this with `filter` aggs, but the workaround
is hacky and probably less performant.
- Adjusts documentation
- Adds a barebones AggregatorTestCase for bucket_script
- Tweaks AggTestCase to use getMockScriptService() for reductions and
pipelines. Previously pipelines could just pass in a script service
for testing, but this didnt work for regular aggs. The new
getMockScriptService() method fixes that issue, but needs to be used
for pipelines too. This had a knock-on effect of touching MovFn,
AvgBucket and ScriptedMetric
3df1c76f9b