`ingress-auth` relation does not work for cross-model relations, needs `namespace` property in schema

When creating the ingress-auth EnvoyFilter, istio-pilot [renders](https://github.com/canonical/istio-operators/blob/110ef7d1f1342a912e5c8e81e3dafffb4acd5da6/charms/istio-pilot/src/charm.py#L165 self.model.name) as the namespace for the auth service in this template. This only works if the ingress-auth service is in the same namespace as istio-pilot.

To fix, the ingress-auth schema will need to be updated to include a namespace property (similar to the ingress schema). Not sure if there's more to fix or tests that should be added to confirm things work.