This PR moves kNN search and dense vector support out of an xpack plugin and
into server.
In #87625 we plan to integrate ANN search into the main `_search` endpoint as a
new top-level component called `knn`. So kNN will be a dedicated part of the
search request, and we'll have kNN logic within the search phases. The classes
and logic will live in server, matching the other search components like
suggesters, field collapsing, etc.
Currently we only test a small subset of cross cluster search functionality in
rest tests living in the 'multi-cluster-search' qa module. In order to increase
test coverage for basic CCS functionality , this change adds a new qa modula
that re-uses a subset of existing yaml rest tests and runs them in a slightly
modified fashion in a CCS scenario.
Document data and other write operations are executed agains a "remote"
cluster, while all calls to the search API aand other APIs that support CCS are
performed on a local cluster connected to the remote with all the data via CCS.
Relates to #84481
Julie Tibshirani
Christoph Büscher