Opensearch 2.x support and misc API improvements

• add opensearch 2 as a supported version, mark search_after as supported by opensearch 2.x
• new usable extension property on ClusterStatus that is true if yellow or green.
• add an example kts script and document how to use kt-search with kts scripts
• add parseHit on SearchResponse
• add ids extension function to quickly get the ids out of the search response
• make the SearchDSL block optional so you can search for everything
• add a variant of indexDocument that serializes an object
• much improved documentation