PoC DO NOT MERGE - Use root object for storing field inference

[carlosdelest]

PoC demoed in GenAI meeting 01/11/2023.

Modifies #9 by creating a root level object for storing inference results.

It also adds a crude source exclude mechanism so the root field is hidden from the user when source fetching is not specified.

For the following index:

PUT test-semantic
{
    "mappings": {
        "properties": {
            "infer_field": {
                "type": "semantic_text",
                "model_id": "my-elser-model"
            },
            "non_infer_field": {
                "type": "text"
            },
            "another_infer_field": {
                "type": "semantic_text",
                "model_id": "my-elser-model"
            }
        }
    }
}

A document ingestion like:

PUT test-semantic/_doc/doc1
{
    "infer_field": "these are not the droids you're looking for",
    "non_infer_field": "he can go about his business",
    "another_infer_field": "move along"
}

will look like this once ingested:

GET test-semantic/_doc/doc1?_source_includes=*
{
  "_index": "test-semantic",
  "_id": "doc1",
  "_version": 1,
  "_seq_no": 0,
  "_primary_term": 1,
  "found": true,
  "_source": {
    "_semantic_text_inference": {
      "infer_field": {
        "lucas": 0.05212344,
        "ty": 0.041213956,
        "dragon": 0.50991,
        "type": 0.23241979,
        "dr": 1.9312073,
        "##o": 0.2797593,
        "these": 1.1422911,
        "robot": 0.3821148
      },
      "another_infer_field": {
        "play": 0.32950363,
        "hum": 0.003634397,
        "tempo": 0.15531318,
        "anger": 0.028992891,
        "soon": 0.37861174,
        "crawl": 0.10688804,
        "road": 0.10052208,
        "action": 0.18112859,
        "talk": 0.48378038,
        "up": 0.17651497,
        "moving": 1.8461642
      }
    },
    "infer_field": "these are not the droids you're looking for",
    "another_infer_field": "move along",
    "non_infer_field": "he can go about his business"
  }
}

Root level field is hidden automatically unless source fetching is specified:

GET test-semantic/_doc/doc1
{
  "_index": "test-semantic",
  "_id": "doc1",
  "_version": 1,
  "_seq_no": 0,
  "_primary_term": 1,
  "found": true,
  "_source": {
    "infer_field": "these are not the droids you're looking for",
    "another_infer_field": "move along",
    "non_infer_field": "he can go about his business"
  }
}