[FEATURE] DocSort to have its own type

[lflores-apex]

Is your feature request related to a problem?

There is no DocSort as of now. If you build a DocSort you are building a ScoreSort.

What solution would you like?

DocSort to have its own type.

What alternatives have you considered?

None

Do you have any additional context?

opensearch-java/java-client/src/main/java/org/opensearch/client/opensearch/_types/SortOptionsBuilders.java

Lines 50 to 54 in eab4091

	* Creates a builder for the {@link ScoreSort _doc} {@code SortOptions} variant.
	*/
	public static ScoreSort.Builder doc() {
	return new ScoreSort.Builder();
	}

https://www.elastic.co/guide/en/elasticsearch/reference/current/sort-search-results.html

The sort is defined on a per field level, with special field name for _score to sort by score, and _doc to sort by index order.