Support filtering elements by metadata

the ListElements endpoint should support 2 new filters:

• metadata_key=XXX to specify the name of a metadata
• metadata_value=YYY to specify the exact value of a metadata

A user can either specify:

• metadata_key only: all elements with that metadata will be listed
• metadata_key + metadata_value: all elements with that metadata + value will be listed

Do NOT support comparison at this stage.