command: add content-disposition flag.

[denizsurmeli]

This PR adds --content-disposition flag to the cp and mv command, which are used for uploading files.

Fixes #569 .

[seruman]

Can you add tests please? You may use content type assertions as a reference;

s5cmd/e2e/util_test.go

Lines 570 to 574 in af8d725

	if opts.contentType != nil {
	if diff := cmp.Diff(opts.contentType, output.ContentType); diff != "" {
	return fmt.Errorf("content-type of %v/%v: (-want +got):\n%v", bucket, key, diff)
	}
	}

[igungor]

@denizsurmeli please update the changelog.

[seruman]

Not related to this PR;
https://github.com/peak/s5cmd/actions/runs/5608257848/jobs/10260392004

#582 introduced flakiness on Windows on cp tests.