Switch to a low-overhead way of authenticating S3 integration

[cw75]

Describe your changes and why you are making these changes

Previously we were doing a list operation to check if the user has access to the bucket, which can be very costly if the bucket has lots of files. This low-overhead approach is borrowed from here and I verified that it works.

Related issue number (if any)

Checklist before requesting a review

• I have created a descriptive PR title. The PR title should complete the sentence "This PR...".
• I have performed a self-review of my code.
• I have included a small demo of the changes. For the UI, this would be a screenshot or a Loom video.
• If this is a new feature, I have added unit tests and integration tests.
• I have run the integration tests locally and they are passing.
• I have run the linter script locally (See python3 scripts/run_linters.py -h for usage).
• All features on the UI continue to work correctly.
• Added one of the following CI labels:
  • run_integration_test: Runs integration tests
  • skip_integration_test: Skips integration tests (Should be used when changes are ONLY documentation/UI)

[saurav-c]

LGTM