feat: marshal pathItem and operation extentions

[prgres]

With entgo and entoas there is no easy way to attach custom extension to the ogen.Spec structure.

x-gen extentions logic was implemented here:

• Proposal: Allow grouping operations into separate handlers #1207
• feat(gen): add support for x-ogen-operation-group extension #1222

Addresses #1228

[tdakkota]

Imports should be fixed and debug print should be removed.

It would be nice to have test to ensure that extensions are actually present in resulting JSON.

Anything else looks good to me.

[prgres]

@tdakkota concerns addressed 💅🏼

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 12 lines in your changes missing coverage. Please review.

Project coverage is 73.90%. Comparing base (31300cd) to head (c830164).
Report is 14 commits behind head on main.

❗ Current head c830164 differs from pull request most recent head 4024c9f

Please upload reports for the commit 4024c9f to get more accurate results.

Files	Patch %	Lines
spec.go	72.72%	8 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1269      +/-   ##
==========================================
+ Coverage   73.87%   73.90%   +0.03%     
==========================================
  Files         189      189              
  Lines       12807    12855      +48     
==========================================
+ Hits         9461     9501      +40     
- Misses       2801     2805       +4     
- Partials      545      549       +4     

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