Implement content type propagation

[pjbull]

Implements propagating content types to all of the cloud providers. Does so by adding a kwarg to all clients content_type_method that by default is set to the built-in mimetypes.guess_type.

Includes tests and documentation.

Changes looks bigger than it is because all of our Local testing resources and Mocks had to be updated to track content types that were set so that we could test them.

Closes #222

[github-actions]

🚀 Deployed on https://deploy-preview-226--gallant-agnesi-5f7bb3.netlify.app

[codecov-commenter]

Codecov Report

Merging #226 (4fac4a0) into master (85268c8) will decrease coverage by 0.1%.
The diff coverage is 93.1%.

@@           Coverage Diff            @@
##           master    #226     +/-   ##
========================================
- Coverage    94.8%   94.7%   -0.2%     
========================================
  Files          21      21             
  Lines        1256    1287     +31     
========================================
+ Hits         1191    1219     +28     
- Misses         65      68      +3     

Impacted Files	Coverage Δ
cloudpathlib/local/localclient.py	96.4% <87.5%> (-1.1%)	⬇️
cloudpathlib/s3/s3client.py	95.4% <90.9%> (-0.7%)	⬇️
cloudpathlib/azure/azblobclient.py	94.2% <92.8%> (-0.5%)	⬇️
cloudpathlib/client.py	87.9% <100.0%> (+0.4%)	⬆️
cloudpathlib/gs/gsclient.py	94.4% <100.0%> (+0.2%)	⬆️

[pjbull]

Thanks @jayqi for review! Integrated all your comments.

[jayqi]

Looks good, aside from small typo. I think you can bump the version in setup.py and then release after merging.