[upload] The upload feature should support uploading multiple files

[pmoravec]

A requirement originating from #3746 (comment): sos upload should allow uploading multiple files.

I prefer implementing this enhancement as a PR separate from the Jose's or Jake's ones, since its implementation will depend on the final class hierarchy and I prefer "layered" approach in adding functionality - it is better for PR authors and reviews to keep PRs smaller.

sos upload FILE1 FILE2 .. FILEn should determine a target / protocol / URL / credentials / .., authenticate there and then simply upload one file after another.

How to deal when say 3rd file out of five will fail to upload? Shall we continue in upload of remaining files or abort? (in either case, we must explicitly list all the files that failed to be uploaded)

And should we allow concurrent files upload..?

[jcastill]

sos upload FILE1 FILE2 .. FILEn should determine a target / protocol / URL / credentials / .., authenticate there and then simply upload one file after another.

How to deal when say 3rd file out of five will fail to upload? Shall we continue in upload of remaining files or abort? (in either case, we must explicitly list all the files that failed to be uploaded)

Lets say we have 5 files and 1 or 2 fail. If the failure is due to an issue in the remote target, and not because it's an empty file, or a file that doesn't exist, then I think we should, in order:
1- Attempt to upload the file N times before failing completely (with a very low N, and a time between retries)
2- List the files that uploaded successfully and the ones that failed at the end.

It's more useful for our support guys to have something to start the analysis, than having to wait for 5 files because a number of them couldn't be uploaded.

And should we allow concurrent files upload..?

Depends on the target. I'll have a chat with our customer portal team to see what they allow and prefer.
Anyway I'd like to work on this PR later on if nobody opposes.