Hybrid Retriever POC - DO NOT MERGE

[Mikep86]

Adds a hybrid retriever for simple hybrid search across lexical & semantic text fields:

PUT wiki-index
{
  "mappings": { 
    "properties": {
      "title": {
        "type": "text"
      },
      "content": {
        "type": "text",
        "fields": {
          "semantic": {
            "type": "semantic_text"
          }
        }
      }
    }
  }
}

POST wiki-index/_search
{
  "retriever": {
    "hybrid": {
      "fields": ["content", "content.semantic"],
      "query": "foo"
    }
  }
}

Semantic reranking using the text_similarity_reranker is integrated:

POST wiki-index/_search
{
  "retriever": {
    "hybrid": {
      "fields": ["content", "content.semantic"],
      "query": "foo",
      "rerank": true,
      "rerank_inference_id": "my-reranker-service",
      "rerank_field": "content"
    }
  }
}

You can use the caret notation to boost matches in certain fields:

POST wiki-index/_search
{
  "retriever": {
    "hybrid": {
      "fields": ["content", "content.semantic^3"],
      "query": "foo"
    }
  }
}

And you can use query_settings to customize the query run against certain fields:

POST wiki-index/_search
{
  "retriever": {
    "hybrid": {
      "fields": ["content", "content.semantic"],
      "query": "foo bar",
      "query_settings": {
        "content": {
          "type": "match",
          "operator": "and"
        }
      }
    }
  }
}

POST wiki-index/_search
{
  "retriever": {
    "hybrid": {
      "fields": ["content", "content.semantic"],
      "query": "foo bar",
      "query_settings": {
        "content": {
          "type": "match_phrase",
          "slop": 1
        }
      }
    }
  }
}

[kderusso]

This is a great POC! As it's a POC I didn't review the code itself, but I left one suggestion to think about when we move to production implementation.

One thing that would be really compelling as an example, is to generate a really complex query using the linear retriever, and then generate the same (nicer, smaller) query using the hybrid retriever.

[kderusso]

I like that we incorporated rerank into this POC - I'd also like to see rule incorporated as I think business rules will be a critical part of the hybrid retriever. In that vein I wonder if there's something more generic we can do with the retrievers that we call - Something like this, that could be easily extended as we add any additional future retrievers or want more customization.

POST wiki-index/_search
{
  "retriever": {
    "hybrid": {
      "fields": ["content", "content.semantic"],
      "query": "foo",
      "rank_modifiers": [
      "rule": { 
         ... 
       },
      "rerank": {
        "inference_id": "my-reranker-service",
        "field": "content"
      }]
    }
  }
}