api: add experimental Gateway API flags

[zhaohuabing]

What type of PR is this?
api: add experimental Gateway API flags

api for #5800, this unblocks us from implementing the GatewayAPI experimental API such as XListenerSets.

[netlify]

✅ Deploy Preview for cerulean-figolla-1f9435 canceled.

Name	Link
🔨 Latest commit	9708a0e
🔍 Latest deploy log	https://app.netlify.com/projects/cerulean-figolla-1f9435/deploys/696d91b67a04c700089fd330

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.85%. Comparing base (5028ac5) to head (9708a0e).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
api/v1alpha1/envoygateway_helpers.go	0.00%	6 Missing ⚠️

❌ Your patch check has failed because the patch coverage (0.00%) is below the target coverage (60.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7970      +/-   ##
==========================================
+ Coverage   72.81%   72.85%   +0.04%     
==========================================
  Files         237      237              
  Lines       35475    35515      +40     
==========================================
+ Hits        25832    25876      +44     
+ Misses       7803     7800       -3     
+ Partials     1840     1839       -1     

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