YAN-1265: pass queried metadata keys to update_dataproduct_list

YAN-1265: takes the key_pair sent as part of a dataproductsearch and passes it to update_dataproduct_list. That function then uses it to add the metadata_key's value (if it exists) to the returned JSON data

For example: if a key_pair of "obscore.dataproduct_type:MS" is sent to dataproductsearch like so:

{ "start_date": "2001-12-12", "end_date": "2032-12-12", "key_pair": "obscore.dataproduct_type:MS" }

Then the dataproductsearch will not only find matches with that key (as it did before), but will return that metadata as part of the returned JSON.

Added in preparation for making this visible on the client side in a future MR (YAN-1263).