feat: add WithServer option

[haysons]

• Add WithServer function in options.go
• Add appendExistHTTPServer function to Graceful struct in graceful.go
• Add tests for TestWithServer functions in graceful_test.go

I hope for a more convenient and comprehensive way to customize http.Server.
Configuring each property of http.Server individually can be quite cumbersome,
and passing both gin.Engine and http.Server into Graceful aligns well with Graceful's intended purpose.

[cedric-appdirect]

Indeed, should have been an option from the beginning. Good catch.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 76.98%. Comparing base (00fab28) to head (0b897ff).
Report is 19 commits behind head on master.

Files with missing lines	Patch %	Lines
options.go	75.00%	2 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   76.49%   76.98%   +0.48%     
==========================================
  Files           2        2              
  Lines         234      252      +18     
==========================================
+ Hits          179      194      +15     
- Misses         37       39       +2     
- Partials       18       19       +1     

Flag	Coverage Δ
go-	?
go-1.20	76.98% <83.33%> (+0.48%)	⬆️
go-1.21	76.98% <83.33%> (+0.48%)	⬆️
go-1.22	76.98% <83.33%> (?)
go-1.23	76.98% <83.33%> (?)
macos-latest	?
ubuntu-latest	76.98% <83.33%> (+0.48%)	⬆️

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