Only the last EnvoyProxy will be used as the OIDC name doesn't include the cluster index

[zhaohuabing]

Rethinking on this, since EnvoyProxy is an optional parameter, its name may not be suitable to be used as part of the generated cluster name. We can always use EndpointRoutingType for OIDC, ExtProc, and other none-route backend clusters and don't need to use the RoutingType in the EnvoyProxy.

Discussed this with @arkodg and @guydc on the EG meeting this week, we still want to respect the routingType in EnvoyProxy configuration for Service Mesh integration and LB policy.

Originally commented by @zhaohuabing on: #4707 (comment)