[FEATURE] Support for composing query variants from 3rd party plugins

Is your feature request related to a problem?

Some plugins such as the OpenSearch LTR plugin add additional query variants https://elasticsearch-learning-to-rank.readthedocs.io/en/latest/searching-with-your-model.html#searching-with-ltr.

opensearch-java current cannot support construction of a query containing such a query variant as QueryVariant interface requiring a kind being specified and Query.Kind being a enum, not allowing for variants to be specified in a 3rd party client.

What solution would you like?

There would be a couple possible solutions to this problem. Converting the enum to a type which allows for open extension might be one. Add a custom variant to the enum which itself allows for a flexible query construction might be another.

What alternatives have you considered?

See above

Do you have any additional context?

N/A

Start by reading java-client/src/main/java/org/opensearch/client/opensearch/_types/query_dsl/QueryVariant.java and Query.java, then compare the OpenSearch LTR plugin's query variant requirements. Determine how the current kind handling limits third-party variants; done means a third-party plugin query can be composed through the Java client without breaking existing query variants.