List the fields a GNQL query may use, with a description and the value
types each accepts. Fields your plan does not include are still listed,
with is_restricted set to true. A term on a restricted field is
dropped from your query rather than rejected; POST /v3/gnql/validate
reports the adjusted query. Requires the same entitlement as search.

Use this to drive query suggestions and client-side checks. Whether a
query is accepted is still decided by POST /v3/gnql/validate.