Vector test tools (#128934)

This adds some testing tools for verifying vector recall and latency
directly without having to spin up an entire ES node and running a rally
track.

Its pretty barebones and takes inspiration from lucene-util, but I
wanted access to our own formats and tooling to make our lives easier.

Here is an example config file. This will build the initial index, run
queries at num_candidates: 50, then again at num_candidates 100 (without
reindexing, and re-using the cached nearest neighbors).

```
[{
  "doc_vectors" : "path",
  "query_vectors" : "path",
  "num_docs" : 10000,
  "num_queries" : 10,
  "index_type" : "hnsw",
  "num_candidates" : 50,
  "k" : 10,
  "hnsw_m" : 16,
  "hnsw_ef_construction" : 200,
  "index_threads" : 4,
  "reindex" : true,
  "force_merge" : false,
  "vector_space" : "maximum_inner_product",
  "dimensions" : 768
},
{
"doc_vectors" : "path",
"query_vectors" : "path",
"num_docs" : 10000,
"num_queries" : 10,
"index_type" : "hnsw",
"num_candidates" : 100,
"k" : 10,
"hnsw_m" : 16,
"hnsw_ef_construction" : 200,
"vector_space" : "maximum_inner_product",
"dimensions" : 768
}
]
```

To execute:

```
./gradlew :qa:vector:checkVec --args="/Path/to/knn_tester_config.json"
```

Calling `./gradlew :qa:vector:checkVecHelp` gives some guidance on how
to use it, additionally providing a way to run it via java directly
(useful to bypass gradlew guff).

server/src/main/java/module-info.java

@@ -479,4 +479,6 @@ module org.elasticsearch.server {
     exports org.elasticsearch.lucene.util.automaton;
     exports org.elasticsearch.index.codec.perfield;
     exports org.elasticsearch.lucene.search;
+    exports org.elasticsearch.index.codec.vectors to org.elasticsearch.test.knn;
+    exports org.elasticsearch.index.codec.vectors.es818 to org.elasticsearch.test.knn;
 }