Make engine top level field mapping parameter

[jmazanec15]

Description

Currently, the engine is included in the method subsection of the knn_vector mapping:

{
  "mappings": {
    "properties": {
      "my_vector": {
        "type": "knn_vector",
        "dimension": 4,
        "method": {
          "name": "hnsw",
          "engine": "faiss",
          "parameters": {}
        }
      }
    }
  }
}

The reasoning for this was to include ann index structure details in the method definition.

However, Im wondering if we should make engine a top-level mapping parameter, as we did with the space_type a couple releases ago:

{
  "mappings": {
    "properties": {
      "my_vector": {
        "type": "knn_vector",
        "dimension": 4,
        "engine": "faiss",
        "method": {
          "name": "hnsw",
          "parameters": {}
        }
      }
    }
  }
}

This will simplify the handling of the method and its parameters, so that it just includes the algorithm and its parameters. This will also make it much easier to add an engine extension point as well.