Start a resumable folder upload into the workspace [write]

For folders over the single-request import's 25 MB: declare the folder's name, the gzipped tar's size (up to 2 GiB) and sha256, then send it in parts and complete. Answers the upload; its parts are numbered from 0. Or send {name, file_id}: a gzipped tar already stored as one of your files, which needs no parts and answers state verified, so complete unpacks it straight away (404 for a file you cannot see, 400 not_downloadable). 404 when resumable imports are not enabled for the workspace, 400 invalid_name or invalid_request (size, sha256 or part_size out of range).