Add `default_model_id` to fields

[esmarkowski]

Setting a default model on an index or field

• Eliminates the need to pass model_id to neural search types

PUT /_search/pipeline/default_model_pipeline 
{
  "request_processors": [
    {
      "neural_query_enricher" : {
        "default_model_id": "bQ1J8ooBpBj3wT4HVUsb",
        "neural_field_default_id": {
           "my_field_1": "uZj0qYoBMtvQlfhaYeud",
           "my_field_2": "upj0qYoBMtvQlfhaZOuM"
        }
      }
    }
  ]
}