Support SDS ingress sidecar for Egress Gateway deployment

@Tao_Li @wenchenglu @skydoctor WRT to https://github.com/istio/istio/issues/14039.

Forking this off from the original megathread for Egress Gateway enhancements, so that we can continue discussing SDS support for Egress Gateway here.
From the issue I can see @Tao_Li you assigned the task to yourself. Is this something you are definitely planning to add support for in the future istio release ?

Would love to keep the ball rolling for this one!!

Original thread message

Unfortunately we have something more urgent coming up which means we don’t have bandwidth for now. Is this something you guys can help and we can provide guidance?

Thanks.

@Tao_Li We are definitely interested in helping out. I will discuss the logistics with my team and get back on this shortly.