Allow pre-signing multipart uploads

[holly-hacker]

Using the Minio SDK on a server, I want to give a client the needed URL(s) to upload large files to an S3-compatible service. It's possible to do this for smaller files through presignedPutObject but there doesn't seem to be a way to presign the uploadPart API required for multipart uploads for large files.

Is this planned? Would it be possible to add an API that allows presigning arbitrary operations?