Upload part size as S3 parameter instead of constant value

Some S3 providers need a custom upload part size (500 MB static value in Nextcloud).
Here is a commit to change this value via S3 configuration, instead of using S3_UPLOAD_PART_SIZE constant.
A new parameter is added for an S3 connection : uploadPartSize

Signed-off-by: Florent <florent@coppint.com>