On multipart upload support

[Creling]

Hello root-gg:

Will you intend to add multipart upload support to plik?

In despite of plik supporting file size up to 10GB, I can hardly upload big files successfully due to the unstable network.

Furthermore, we need to set keepalive_timeout to a large value if we want to upload big files, which will bring risks.

If plik could support multipart upload, above problems will be alleviated.

Thank you.

Creling