refactor: clean envoygateway validate methods

[shawnh2]

What type of PR is this?

What this PR does / why we need it:

Which issue(s) this PR fixes:

Prepare for #3159, this part is very likely to get conflicts.

[codecov]

Codecov Report

Attention: Patch coverage is 88.76404% with 10 lines in your changes missing coverage. Please review.

Project coverage is 67.67%. Comparing base (c2c705b) to head (7203618).

Files	Patch %	Lines
api/v1alpha1/validation/envoygateway_validate.go	88.76%	8 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3997      +/-   ##
==========================================
+ Coverage   67.59%   67.67%   +0.07%     
==========================================
  Files         183      183              
  Lines       22446    22471      +25     
==========================================
+ Hits        15173    15207      +34     
+ Misses       6193     6185       -8     
+ Partials     1080     1079       -1     

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[shawnh2]

/retest