(feat) Implement SourceUnitEnumChunker for S3 source

[abmussani]

Description:

This PR implements SourceUnitEnumChunker for S3 source. Each Object in the bucket will be considered as Unit.

Checklist:

• Tests passing (make test-community)?
• Lint passing (make lint this requires golangci-lint)?

[rosecodym]

requesting changes until we have an internal discussion about unit granularity

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Abdul Basit seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}