allow cat to take part by part numbers

[harshavardhana]

Community Contribution License

All community contributions in this pull request are licensed to the project maintainers under the terms of the Apache 2 license. By creating this pull request I represent that I have the right to license the contributions to the project maintainers under the Apache 2 license.

Description

allow cat to take part by part numbers

Motivation and Context

this allows for downloading different parts by part numbers.

How to test this PR?

Upload a multipart object and then specify

mc cat --part-number=2 alias/bucket/bigfile

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Optimization (provides speedup with no functional changes)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] Fixes a regression (If yes, please add commit-id or PR # here)
• [ ] Unit tests added/updated
• [ ] Internal documentation updated
• [ ] Create a documentation update request here

[harshavardhana]

LGTM, but just curious about when you want to use this. It looks like we can find the number of parts from the Etag?

It is useful when you want to download specific part numbers, to test "decryption" per part. It was used in a customer environment to uncover a bug.