Update data structure and logic to handle both Swagger 2 and OpenAPI 3

[ndeloof]

Change type

🆕 Add support for Open API 3.x

Short description

This PR introduce support for Open API 3.x document

Fixes

fixes #21

Full description

Add required attributes so the same codebase can be used to parse swagger 2.x and openapi 3.x documents.
This is used in https://github.com/allons-y/openapi (go-swagger fork) as a proof of concept
Let me know if this makes sense, then I'll adapt tests so they are executed and adapted for both file formats

Checklist

• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squashed my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.

[fredbi]

Thank you. I’ll take a thorough look at it and at your fork as well. We might have a to discuss a few things. I’ll let you know shortly

[ndeloof]

updated: added a 2nd commit to run spec_test with swagger and openapi files, to confirm both are well supported

[codecov]

Codecov Report

❌ Patch coverage is 38.35616% with 180 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.69%. Comparing base (c14c0a6) to head (68a6c45).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
expander.go	50.40%	43 Missing and 18 partials ⚠️
components.go	14.51%	49 Missing and 4 partials ⚠️
media_type.go	23.21%	35 Missing and 8 partials ⚠️
request_body.go	31.81%	10 Missing and 5 partials ⚠️
properties.go	71.42%	3 Missing and 3 partials ⚠️
parameter.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #234      +/-   ##
==========================================
- Coverage   67.51%   63.69%   -3.82%     
==========================================
  Files          30       34       +4     
  Lines        2395     2749     +354     
==========================================
+ Hits         1617     1751     +134     
- Misses        615      791     +176     
- Partials      163      207      +44     

Flag	Coverage Δ
oldstable-macos-latest	63.10% <38.35%> (-3.81%)	⬇️
oldstable-ubuntu-latest	63.10% <38.35%> (-3.81%)	⬇️
oldstable-windows-latest	63.65% <38.35%> (-3.84%)	⬇️
stable-macos-latest	63.10% <38.35%> (-3.81%)	⬇️
stable-ubuntu-latest	63.10% <38.35%> (-3.81%)	⬇️
stable-windows-latest	63.65% <38.35%> (-3.84%)	⬇️

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