Support TransferConfig for s3 uploads and downloads

[ejm714]

Adds the ability to specify boto3.s3.transfer.TransferConfig parameters, which is useful for controlling multipart and thread use in uploads and downloads.

Outstanding:

• test coverage

Closes #147

[github-actions]

🚀 Deployed on https://deploy-preview-150--gallant-agnesi-5f7bb3.netlify.app

[codecov]

Codecov Report

Merging #150 (3942ac7) into master (25ff783) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff           @@
##           master    #150   +/-   ##
======================================
  Coverage    93.9%   93.9%           
======================================
  Files          21      21           
  Lines        1177    1179    +2     
======================================
+ Hits         1106    1108    +2     
  Misses         71      71           

Impacted Files	Coverage Δ
cloudpathlib/s3/s3client.py	93.1% <100.0%> (+0.1%)	⬆️

[pjbull]

@ejm714 Got a "live" test running with the code that you provided. Here's the diff if you are curious:
https://github.com/drivendataorg/cloudpathlib/pull/150/files/eee669a01883f61fd4199234cda183bb9a5fe22f..3942ac70e3d9e78d446efa0ac5d4cfbf13af3059

• Bonus fix: give the test rigs a live_server flag so that we can branch within tests if necessary (like for this one).

I think this is ready to merge, thanks!