Make NKG-specific Gateway validation consistent with HTTPRoute

[pleshakov]

This PR #455 brought NKG-specific validation for HTTPRoutes. The implementation uses https://pkg.go.dev/k8s.io/apimachinery/pkg/util/validation/field to generate validation errors.

Make Gateway-related errors consistent with HTTPRoute-related errors (start using that package above)

Aha! Link: https://nginx.aha.io/features/NKG-57