Move kNN search and dense vectors to core

[jtibshirani]

This PR proposes moving 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.

Why can't we leave dense vector in its own plugin, and just add kNN search
logic in server?

• This makes testing more difficult/ confusing, since kNN search can only be
  exercised effectively using dense vector fields. All tests for kNN search would
  need to live in the dense vector plugin, although kNN is part of core search.
• It feels cleanest for all logic related to kNN search to live in the same
  place.

[elasticmachine]

Pinging @elastic/es-search (Team:Search)

[elasticmachine]

Pinging @elastic/clients-team (Team:Clients)

[jtibshirani]

Summary of changes
Scripting:

• ScoreScriptUtils with vector functions renamed to VectorScoreScriptUtils, moved to org.elasticsearch.script
• Rename DenseVectorFunctionTests to VectorScoreScriptUtilsTests
• Merge whitelist extensions (*.txt files) into existing context files
• Move script fields API classes to org.elasticsearch.script.field.vectors
• Move script YAML tests to lang-painless test suite

kNN search:

• Move RestKnnSearchAction to org.elasticsearch.rest.action.search
• Move supporting classes like KnnVectorQueryBuilder to org.elasticsearch.search.vectors
• Move kNN search YAML tests to rest-api-spec test suite

Dense vector field:

• Move field mapper and helper classes to org.elasticsearch.index.mapper.vectors

Other:

• Remove hacky interface PerFieldKnnVectorsFormatFieldMapper since it's no longer needed, also remove MappingLookup#getKnnVectorsFormatForField
• Make sure _knn_search works with new CCS integration tests

[mayya-sharipova]

@jtibshirani Thanks, this arrangement LGTM.