feat: add basic generics support

[ubogdan]

Describe the PR
This is a competition for #1194.

Relation issue
#1170 #1194

Additional context
Finish contribution in order to speed up things.

[codecov]

Codecov Report

Merging #1225 (adf24b0) into master (67cb768) will decrease coverage by 0.15%.
The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master    #1225      +/-   ##
==========================================
- Coverage   94.96%   94.80%   -0.16%     
==========================================
  Files          12       14       +2     
  Lines        2520     2600      +80     
==========================================
+ Hits         2393     2465      +72     
- Misses         69       74       +5     
- Partials       58       61       +3     

Impacted Files	Coverage Δ
generics_other.go	50.00% <50.00%> (ø)
packages.go	87.30% <80.00%> (+0.56%)	⬆️
generics.go	90.47% <90.47%> (ø)
operation.go	96.66% <100.00%> (+0.02%)	⬆️

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[ubogdan]

Unfortunately, this implementation doesn't produce a valid OAS2 definition.

[ubogdan]

LGTM 😄