Title: Ability to set minimum number of hosts per-zone when using Zone Aware Routing

Description:
In addition to setting minClusterSize, I'd like the ability to set the minimum number of upstream hosts in the local zone in order for zonal routing to be performed.

Relevant Links:

• https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/upstream/load_balancing/zone_aware#arch-overview-load-balancing-zone-aware-routing
• https://www.envoyproxy.io/docs/envoy/latest/configuration/upstream/cluster_manager/cluster_runtime#zone-aware-load-balancing