Adds customizable CORS configurations

[TerryMooreII]

Adds functionality from Issue #51, test via unit tests and via deployment to our lambda

[codecov-commenter]

Codecov Report

Merging #52 into master will increase coverage by 1.73%.
The diff coverage is 95.00%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   87.43%   89.16%   +1.73%     
==========================================
  Files           5        6       +1     
  Lines         207      277      +70     
  Branches       62       85      +23     
==========================================
+ Hits          181      247      +66     
- Misses         26       30       +4     

Impacted Files	Coverage Δ
lib/cors.ts	94.66% <94.66%> (ø)
lib/proxyIntegration.ts	87.75% <100.00%> (-0.60%)	⬇️

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