Add knn search parameter and remove knn query

[puppylpg]

KnnQuery and KnnSearch are two different classes in elasticsearch java client and are for different queries:

• KnnSearch: is the top level knn query in the elasticsearch request;
• KnnQuery: is the knn query inside query clause;

They both have different parameters and different classes in elasticsearch java client. However, in the current spring data elasticsearch implementation, they are mixed together and are all converted into KnnSearch finally.

So I wanna submit a PR to:

1. remove KnnQuery in NativeQuery: and I think this clause should be constructed inside the co.elastic.clients.elasticsearch._types.query_dsl.Query in NativeQuery by user manually;
2. add withKnnSearches method in NativeQueryBuilder: this may be a loss without which KnnSearch can't be constructed by user;
3. add knn search mapping parameters in elasticsearch 8;

If there are questions about the issues above, we can discuss further.