List Search filters and output fields

Every name accepted by POST /v1/search. filter_ops lists the operators a name accepts in filters (an empty list means output-only), and returnable says whether it may be named in fields. Includes type, unit, description, interpretation hints, null meaning, source and the Search price. No authentication is required. The response carries an ETag and Cache-Control: public, max-age=3600; send If-None-Match to receive 304 when nothing changed.