This commit adds a `rewrite` parameter to the validate API in order to shown
how the given query is re-written into primitive queries. For example, an MLT
query is re-written into a disjunction of the selected terms. Other use cases
include `fuzzy`, `common_terms`, or `match` query especially with a
`cutoff_frequency` parameter. Note that the explanation is only given for a
single randomly chosen shard only, so the output may vary from one shard to
another.
Relates #1412Closes#10147
c347dfe91c