[7.x] Add multi_terms aggs (#67597) (#68490)

Adds a multi_terms aggregation support. The multi terms aggregation works
very similarly to the terms aggregation but supports multiple terms. The goal
of this PR is to add the basic functionality so it is not optimized at the
moment. It will be done in follow up PRs.

Closes #65623

Author: imotov

docs/reference/aggregations/bucket.asciidoc

@@ -50,6 +50,8 @@ include::bucket/iprange-aggregation.asciidoc[]
 
 include::bucket/missing-aggregation.asciidoc[]
 
+include::bucket/multi-terms-aggregation.asciidoc[]
+
 include::bucket/nested-aggregation.asciidoc[]
 
 include::bucket/parent-aggregation.asciidoc[]