In order to ensure that transform extensions aren't modifying metadata they're not supposed to, the host must have a location to store the upstream metadata prior to sending the document to the extension to transform.

This ticket should add a field upstreamMetadata to QueryExecutionContextBase and update QueryExecutionContext and QueryExecutionContextAdapter. By not updating the api.h or QueryExecutionContextHandle, the extension will not have access to this field, only the host will.

class QueryExecutionContextBase { 
public:     
     // Existing methods.     
     DocumentMetadataFields upstreamMetadata; 
};