fix(o2k): param-explode default true if form style

[Tieske]

"explode" defaulted to false.
When the style is "form" the "explode" default is now true, in all
other cases false.

OAS ref: https://swagger.io/specification/v3/#parameter-object (2nd table in this paragraph)

FTI-6088

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.77%. Comparing base (b151569) to head (be5fe2a).

Files	Patch %	Lines
openapi2kong/validator.go	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #186      +/-   ##
==========================================
+ Coverage   67.76%   67.77%   +0.01%     
==========================================
  Files          23       23              
  Lines        2491     2492       +1     
==========================================
+ Hits         1688     1689       +1     
  Misses        643      643              
  Partials      160      160              

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