Implement support for multiple HTTP success statuses

[haimgel]

When OutputWithHTTPStatus is implemented on the output port, HTTPStatus() defines the successful HTTP status of a given operation, and ExpectedHTTPStatuses() defines all expected HTTP statuses for API documentation purposes.

See #153 for the discussion about this feature.

[haimgel]

@vearutop, any feedback is very welcome! In particular, I wasn't quite sure about the naming of this new interface and its methods, and which package exactly it should be.

[codecov[bot]]

Codecov Report

Merging #155 (794fde0) into master (0e69b49) will decrease coverage by 0.02%. The diff coverage is 56.25%.

@@            Coverage Diff             @@
##           master     #155      +/-   ##
==========================================
- Coverage   77.68%   77.67%   -0.02%     
==========================================
  Files          28       28              
  Lines        1600     1608       +8     
==========================================
+ Hits         1243     1249       +6     
- Misses        235      236       +1     
- Partials      122      123       +1     

Flag	Coverage Δ
unittests	77.67% <56.25%> (-0.02%)	:arrow_down:

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Impacted Files	Coverage Δ
response/encoder.go	66.94% <50.00%> (+0.42%)	:arrow_up:
openapi/collector.go	64.13% <60.00%> (-0.08%)	:arrow_down:

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[vearutop]

Thank you, looks good! 👍