Nullable Custom Resource ResponseURL

[talenfisher]

• ResponseURL is now an optional argument to custom resource requests, to allow for easier testing.
• The lambda output for Custom Resources is now the full response that would've been sent to CloudFormation, rather than just the output data

[codecov]

Codecov Report

Merging #455 (7511202) into master (072f1ed) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #455      +/-   ##
==========================================
+ Coverage   93.28%   93.29%   +0.01%     
==========================================
  Files          42       42              
  Lines         625      626       +1     
==========================================
+ Hits          583      584       +1     
  Misses         42       42              

Impacted Files	Coverage Δ
src/CustomResource/CustomResourceLambdaHost.cs	97.10% <100.00%> (+0.04%)	⬆️
src/CustomResource/CustomResourceRequest.cs	100.00% <100.00%> (ø)

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