made http retry codes configurable

[kristinapathak]

fixes #175

[codecov]

Codecov Report

Merging #219 into master will increase coverage by 0.10%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
+ Coverage   69.95%   70.06%   +0.10%     
==========================================
  Files           7        7              
  Lines         739      745       +6     
==========================================
+ Hits          517      522       +5     
- Misses        211      212       +1     
  Partials       11       11              

Impacted Files	Coverage Δ
caduceus_type.go	100.00% <ø> (ø)
main.go	0.00% <0.00%> (ø)
outboundSender.go	84.46% <100.00%> (+0.15%)	⬆️
senderWrapper.go	85.88% <100.00%> (+0.34%)	⬆️

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