Add s3 head object function.

[DailyDreaming]

We use this functionality already, this just centralizes it into its own function in an s3 lib.

TODO: What is the "Right Way" to invoke boto3 sessions with proper credential caching?

Changelog Entry

To be copied to the draft changelog by merger:

• Add s3 head object function.

Reviewer Checklist

• Make sure it is coming from issues/XXXX-fix-the-thing in the Toil repo, or from an external repo.
  • If it is coming from an external repo, make sure to pull it in for CI with:

    contrib/admin/test-pr otheruser theirbranchname issues/XXXX-fix-the-thing
    

  • If there is no associated issue, create one.
• Read through the code changes. Make sure that it doesn't have:
  • Addition of trailing whitespace.
  • New variable or member names in camelCase that want to be in snake_case.
  • New functions without type hints.
  • New functions or classes without informative docstrings.
  • Changes to semantics not reflected in the relevant docstrings.
  • New or changed command line options for Toil workflows that are not reflected in docs/running/{cliOptions,cwl,wdl}.rst
  • New features without tests.
• Comment on the lines of code where problems exist with a review comment. You can shift-click the line numbers in the diff to select multiple lines.
• Finish the review with an overall description of your opinion.

Merger Checklist

• Make sure the PR passes tests.
• Make sure the PR has been reviewed since its last modification. If not, review it.
• Merge with the Github "Squash and merge" feature.
  • If there are multiple authors' commits, add Co-authored-by to give credit to all contributing authors.
• Copy its recommended changelog entry to the Draft Changelog.
• Append the issue number in parentheses to the changelog entry.

[adamnovak]

@DailyDreaming The right way to get a global boto3 session is not bound out; we only have module-level client and resource getters. So right now you can either hold your own AWSConnectionManager for its session() function, or you could add something to the session.py module to get at the global AWSConnectionManager's session().

[adamnovak]

This looks good in general, but it needs a docstring and I think we need to quiet down some warnings it adds.

[adamnovak]

This looks OK to me now.