feat: validate HTTPRoute features

[randmonkey]

What this PR does / why we need it:

Validate features in spec of HTTPRoute and reject the HTTPRoutes using unspported features.

Which issue this PR fixes:

fixes #5253

Special notes for your reviewer:

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (00b12e1) 75.9% compared to head (9ea2db1) 76.0%.

Files	Patch %	Lines
internal/admission/validation/gateway/httproute.go	92.0%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5312   +/-   ##
=====================================
  Coverage   75.9%   76.0%           
=====================================
  Files        170     170           
  Lines      19549   19589   +40     
=====================================
+ Hits       14845   14894   +49     
+ Misses      3865    3858    -7     
+ Partials     839     837    -2     

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[mlavacca]

I left some comments in the PR. In addition to them, I'd suggest that we keep the validateHTTPRouteFeatures function for real features validation (timeouts, filters, etc.) and we extract parentRefs group/kind and backendRefs group/kind validation into proper functions, as I do not think they are not proper "features", but instead accepted values for specific fields. What do you think?

[randmonkey]

I left some comments in the PR. In addition to them, I'd suggest that we keep the validateHTTPRouteFeatures function for real features validation (timeouts, filters, etc.) and we extract parentRefs group/kind and backendRefs group/kind validation into proper functions, as I do not think they are not proper "features", but instead accepted values for specific fields. What do you think?

I prefer to extract the validation of ParentRefs in HTTPRoutes out because this should be validated before we fetch the gateway from parentRefs. So we have the reason to extract it out and run it before validating process reaches validateHTTPRouteFeatures which is run after we found the gateway to attach to.

But I think supported kinds of backend is also a "feature". Consider that in some future versions we also support KongServiceFacade or other kinds of backends, I think we should put the validation of supported backend kinds in validating features. Also, we have a rule that filters in backendRefs is not supported, which should also be checked in validating features. Extracting checking of group/kinds will run 2 loops on checking each backendRefs, which does not simplify the validation process.