Refactor GS authentication to use default credentials (#514)

[pjbull]

• Refactor authentication to use default credentials for Google Cloud Storage client

[github-actions]

🚀 Deployed on https://deploy-preview-526--gallant-agnesi-5f7bb3.netlify.app

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.6%. Comparing base (863e884) to head (e82764b).

Files with missing lines	Patch %	Lines
cloudpathlib/gs/gsclient.py	50.0%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #526     +/-   ##
========================================
- Coverage    93.4%   92.6%   -0.9%     
========================================
  Files          23      23             
  Lines        1800    1750     -50     
========================================
- Hits         1682    1621     -61     
- Misses        118     129     +11     

Files with missing lines	Coverage Δ
cloudpathlib/gs/gsclient.py	91.1% <50.0%> (-0.2%)	⬇️

... and 7 files with indirect coverage changes

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