feat: add s3-sync sidecar option (breaking values changes)

[benchoncy]

What issues does your PR fix?

• fixes support loading DAG definitions from S3 buckets #249

What does your PR do?

Note: This PR suggests interface changes with dags.gitSync values. May need to be viewed in the context of #501.

• Generalises the definition of gitSync to a general sync pod definition
• Refactor dags.gitSync values to dags.sync
• Add support for s3 sync under new dags.sync values
• Amend value validation for gitSync and add new validation for s3
• Update README to reflect changes

Checklist

For all Pull Requests

• Commits are signed off
• Commits have semantic messages
• Documentation updated
• Passes ct linting

For releasing ONLY

• Chart.yaml version bumped
• CHANGELOG.md updated

[jdobyns]

Whats the hold up on this?

[stale]

This issue has been automatically marked as stale because it has not had activity in 60 days.
It will be closed in 7 days if no further activity occurs.

Thank you for your contributions.

---

Issues never become stale if any of the following is true:

1. they are added to a Project
2. they are added to a Milestone
3. they have the lifecycle/frozen label

[benchoncy]

@thesuperzapper Considering there is now a competing pull request for the same feature #828, which appears to have gotten more discussion, should I close this pull request? There are now conflicts, so I'm looking to determine if this is worth updating.

[thesuperzapper]

@benchoncy I really do appreciate the effort you have put in.

However, I am exceptionally unlikely to release any breaking changes in the values for the foreseeable future, as it's just too much work for people to migrate.

If you think you can do a better job then the other PR at adding new values specifically for S3 (or something similar), then you're welcome to either update this PR or make a new one.