This commit adds support for histogram and date_histogram agg compound order by refactoring and reusing terms agg order code. The major change is that the Terms.Order and Histogram.Order classes have been replaced/refactored into a new class BucketOrder. This is a breaking change for the Java Transport API. For backward compatibility with previous ES versions the (date)histogram compound order will use the first order. Also the _term and _time aggregation order keys have been deprecated; replaced by _key.
Relates to #20003: now that all these aggregations use the same order code, it should be easier to move validation to parse time (as a follow up PR).
Relates to #14771: histogram and date_histogram aggregation order will now be validated at reduce time.
Closes#23613: if a single BucketOrder that is not a tie-breaker is added with the Java Transport API, it will be converted into a CompoundOrder with a tie-breaker.
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