[FEATURE] Add search_pipeline support to opensearch-java

Is your feature request related to a problem?

I am unable to include a search_pipeline in my queries with the opensearch-java library:

error=Unknown field 'search_pipeline' for type 'org.opensearch.client.opensearch.core.SearchRequest$Builder'

What solution would you like?

It should be possible, in opensearch-java, to use a dynamic search pipeline similar to what would be configured in a REST call with:

},
  "search_pipeline": {
    "phase_results_processors": [{
      "normalization-processor": {
        "normalization": { "technique": "min_max" },
        "combination": {
          "technique": "arithmetic_mean",
          "parameters": {
            "weights": [ 0.9, 0.1 ]
          }
        }
      }
    }]
  }
}

It seems as though this support needs to be added to https://github.com/opensearch-project/opensearch-java/blob/main/java-client/src/main/java/org/opensearch/client/opensearch/core/MsearchRequest.java

What alternatives have you considered?

Our app could use the REST API, but we use opensearch-java for all other requests so we'd like to stay consistent.

Start with java-client/src/main/java/org/opensearch/client/opensearch/core/MsearchRequest.java and the org.opensearch.client.opensearch.core.SearchRequest.Builder referenced by the error. Compare the existing request model with the REST search_pipeline example and determine how dynamic phase-results processors should be represented. Done means callers can express this search pipeline through opensearch-java without falling back to the REST API.