Support @x- attributes for operations

[yutopp]

Describe the PR
Add support @x- attributes for operations in paths section. (https://swagger.io/docs/specification/2-0/swagger-extensions/)

Relation issue
#398. This PR introduces additional supports for it.

Additional context
(none)

Thanks.

[codecov-io]

Codecov Report

Merging #452 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #452      +/-   ##
==========================================
+ Coverage   83.27%   83.36%   +0.09%     
==========================================
  Files           7        7              
  Lines        1459     1467       +8     
==========================================
+ Hits         1215     1223       +8     
  Misses        138      138              
  Partials      106      106

Impacted Files	Coverage Δ
operation.go	86.2% <100%> (+0.27%)	⬆️

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[easonlin404]

Seems like don't need to commit go.mod and go.sum files because no new libraries be imported.

[yutopp]

Thank you for your review. I fixed that. f85ce40

[easonlin404]

LGTM

[easonlin404]

@yutopp Thanks four your contributions.