Allow the entry Gateway's SDS to search all namespaces by certificate secrets.

1. Proposed title of this feature request

Allow Ingress gateway SDS to search all namespaces for certificate secrets.

2. What is the nature and description of the request? 

To make it easier to manage the autonomous teams (customer have several more teams with over a thousand apps), he needs the secret with TLS to be in the app project and be able to reference it in the Gateway.

This need is discussed here: https://github.com/istio/istio/issues/14598

3. Why does the customer need this? (List the business requirements here)

Because they have a lot of teams and thousands apps, to manage the teams easy and better.

4. List any affected packages or components.

Istio SDS Gateway, namespace security