[FEATURE] Support default search pipeline when create index for hybrid search

[PiaopiaoZheng]

Is your feature request related to a problem?

• I use hybrid search including boolean query and neural query.
• And I also use Multi-search to search multi items in one request, but MsearchRequest still doesn't support to set the search pipeline.
• As the Default search pipeline doc mentioned, I could set a default search pipeline with weights when create knn index.
• I can't set the weights for hybrid search if I use Multi-search.

What solution would you like?

Can we support to add default search pipeline in open search client as the following API behavior?

PUT /my_index/_settings 
{
  "index.search.default_pipeline" : "my_pipeline"
}