List all possible custom fields for an entity type

Returns every custom field defined for the given entity type — grouped into sections, each field carrying its fieldGk, name, data type, required/writable flags, and (for SELECT fields) its valid options — with no entity-instance values. Use this to discover which fields exist and what fieldGk/optionGk values to send to PUT .../fields/{fieldGk} before creating or editing an entity's custom field values.