Creates a new folder for organizing audiences in GTM Studio. Folders group related audiences together, making it easier to navigate and manage large collections — for example, grouping by campaign, region, or team. Specify name and optionally set starred to true to mark the folder for quick access. The folder is created empty; audiences are assigned to it via the Create Audience or Update Audience endpoints using folderId. Returns 201 Created with the full folder resource including the system-assigned folderId and creation metadata. Returns 400 if required fields are missing or invalid.