* Feat: extend gateway trait to set class in spec

`kubernetes.io/ingress.class` annotation is deprecated in favor of
`.spec.ingressClassName`. However, there is no way to set it through
the gateway trait for now.

This commit extends the gateway trait by adding `classInSpec` to
parameter. Forcing the use of `.spec.ingressClassName` makes sense, but
some ingress controllers (including old versions) may not recognize
this field. Therefore, set default value of `classInSpec` to `false`
for backward compatibility.
Signed-off-by: Sunghoon Kang <hoon@linecorp.com>

* Chore: update classInSpec usage
Signed-off-by: Sunghoon Kang <hoon@linecorp.com>