Add list CORS configurations endpoint

[mthenw]

What did you implement:

Closes #XXXXX

How did you implement it:

How can we verify it:

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: NO
Is it a breaking change?: NO

[codecov]

Codecov Report

Merging #463 into master will increase coverage by 0.27%.
The diff coverage is 92%.

@@            Coverage Diff            @@
##           master    #463      +/-   ##
=========================================
+ Coverage   68.22%   68.5%   +0.27%     
=========================================
  Files          34      34              
  Lines        2112    2137      +25     
=========================================
+ Hits         1441    1464      +23     
- Misses        611     612       +1     
- Partials       60      61       +1

Impacted Files	Coverage Δ
httpapi/httpapi.go	79.75% <100%> (+0.61%)	⬆️
libkv/cors.go	88.88% <84.61%> (-0.73%)	⬇️

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