Support ANN search by ID

[jmazanec15]

Related issue: opendistro/for-elasticsearch-docs#467:

In the docs it is specified that the knn query can be used to search nearest neighbours.

GET my-knn-index-1/_search
{
  "size": 2,
  "query": {
    "knn": {
      "my_vector2": {
        "vector": [2, 3, 5, 6],
        "k": 2
      }
    }
  }
}

The dimensionality of the data is approximately 1300 embeddings. I was wondering if it is possible to query knn results by passing the id of the ES document instead of the whole vector?

Currently, we do not support this. However, it is an interesting use case. We would need to have a step in the query pipeline where we get the vector based on the id and then pass it on to the rest of the query logic