NoCredentials class for accessing publicly available data

[nlw0]

NoCredentials simply omits the "Authorization" header, and works with publicly available data. Something similar is actually offered in the C++ GCP API.

[codecov-commenter]

Codecov Report

Merging #50 (7298551) into master (adef89e) will increase coverage by 11.82%.
The diff coverage is 100.00%.

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@@             Coverage Diff             @@
##           master      #50       +/-   ##
===========================================
+ Coverage    8.92%   20.75%   +11.82%     
===========================================
  Files          11       11               
  Lines         448      453        +5     
===========================================
+ Hits           40       94       +54     
+ Misses        408      359       -49     

Impacted Files	Coverage Δ
src/GoogleCloud.jl	100.00% <ø> (ø)
src/credentials.jl	2.50% <ø> (+2.50%)	⬆️
src/api/api.jl	55.31% <100.00%> (+33.01%)	⬆️
src/session.jl	7.81% <100.00%> (+7.81%)	⬆️

... and 1 file with indirect coverage changes

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[nlw0]

@mattBrzezinski I have rebased this. The point is that we can now easily write tests without credentials, so here we should already be able to increase a bit test coverage.

[nlw0]

@mattBrzezinski before I merge this, I'm not sure how to do versioning. Is it OK if we leave the current number at master for a while before we add other new features and then later tag a new version?

[mattBrzezinski]

@mattBrzezinski before I merge this, I'm not sure how to do versioning. Is it OK if we leave the current number at master for a while before we add other new features and then later tag a new version?

Yeah you can do that, you don't need to tag everything you release. Just that when you do pick the appropriate SemVer bump for the changes.