Skip parts of a file during an mc od upload with skip=

Documented steps

1. Run mc od if=file.zip of=myaistor/mybucket/file.zip skip=N to skip the first N parts during upload.
2. This is useful for testing upload speed on only a portion of a large multi-part object.
3. Combine with size and/or parts to control part granularity while skipping.
4. Review the elapsed timing or --json output to observe the effect of skipping.