Add support for "Increase ranking based on item context attributes" in the Indexing Connector SDK

[alok-gupta-sada]

Add support in Indexing connector SDK for setting the named attributes associated with the item for influencing the ranking of the item based on the context in the request.

[cjohnson78]

For reference: https://github.com/googleapis/google-api-java-client-services/blob/f2989c09aadd7f7e5f8fe7a940ae112de9fec8f5/clients/google-api-services-cloudsearch/v1/1.31.0/com/google/api/services/cloudsearch/v1/model/ItemMetadata.java#L206