support form tag

[sdghchj]

Describe the PR
This is a completed way to support form tag.
As the extensions of schema only marshal keys with prefix x- into json, so I just add the form tag name with key form into the schema extensions, and look it up when parsing @param.
And this way also doesn't affect the final output docs.

Relation issue
#1422

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04 🎉

Comparison is base (19ddb4d) 85.15% compared to head (e28806e) 85.20%.

❗ Current head e28806e differs from pull request most recent head 3034d07. Consider uploading reports for the commit 3034d07 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1502      +/-   ##
==========================================
+ Coverage   85.15%   85.20%   +0.04%     
==========================================
  Files          19       19              
  Lines        3630     3642      +12     
==========================================
+ Hits         3091     3103      +12     
  Misses        460      460              
  Partials       79       79              

Impacted Files	Coverage Δ
field_parser.go	96.93% <100.00%> (+0.03%)	⬆️
operation.go	96.14% <100.00%> (+0.02%)	⬆️
parser.go	94.52% <100.00%> (+0.02%)	⬆️

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