Mixed FQDN and IP/UDS endpoints when using multiple backendRefs

[guydc]

Description:
Create SecurityPolicy or EnvoyExtensionPolicy with two backendRefs for ext-auth/proc:

• An FQDN Backend
• A K8s Service or Backend with UDS/IP endpoints

The policies are accepted, but envoy rejects the config:
[2025-01-14 00:12:26.958][1][warning][config] [source/extensions/config_subscription/grpc/grpc_subscription_impl.cc:138] gRPC config for type.googleapis.com/envoy.config.endpoint.v3.ClusterLoadAssignment rejected: malformed IP address: foo.br. Consider setting resolver_name or setting cluster type to 'STRICT_DNS' or 'LOGICAL_DNS'

Similar to mixed-endpoint checks within a single destination, EG should verify that endpoints aggregated from multiple destinations are not mixed.

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