Only use anon credentials for public s3 buckets

[cmckeen]

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Fixes #188

Description

Add optional s3_bucket_public setting to use anonymous credentials when accessing public S3 buckets.

Checklist

• [ ] Read the CONTRIBUTING document.
• [ ] Read the CODE OF CONDUCT document.
• [ ] Add tests to cover changes.
• [ ] Ensure your code follows the code style of this project.
• [ ] Ensure CI and all other PR checks are green OR
  • [ ] Code compiles correctly.
  • [ ] Created tests which fail without the change (if possible).
  • [ ] All new and existing tests passed.
• [ ] Add your changes to Unreleased section of CHANGELOG.
• [ ] Improve and update the README (if necessary).
• [ ] Ensure documentation is up-to-date. The same file will be updated in plugin index when your PR is accepted, so it will be available for end-users at http://plugins.drone.io.

[cmckeen]

fixes #188