Add expandNested support to "exact_knn" query clause

[kaushikau]

Description

This is a follow-up to #2826, and adds support for the expandNested query parameter in the exact_knn query. This allows for all child document scores to be retrieved instead of only the highest-scoring one.

Example of query with expandNested:

{
  "size": 2,
  "query": {
    "exact_knn": {
      "my_vector": {
        "vector": [2, 3, 5, 6],
        "space_type": "l2",
        "expand_nested_docs": "true"
      }
    }
  }
}

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[shatejas]

Just for clarification, what case is this handling? I think if it hits this case then docId will already be -1 right? is it safe to return no more docs here?

[shatejas]

will there be a case where docId won't be less than parent? it will either be NO_MORE_DOCS or less than parent right?