Add a file to a data lake

Makes an existing file a member of the lake. Takes no body. Idempotent: adding a file that is already a member succeeds and changes nothing. Outside the restore window below, a caller can add only their own files (or, as the lake's owner, the owner's files); any other file is a 404. Membership alone does not ingest the file - poll GET /api/v1/data-lakes/{id}/files/{file_id} until it is ready before relying on it in search. Re-adding a file within about 30 minutes of removing it restores the membership it had.