upstream: Introducing close_connections_on_host_set_change property (#7675)

Signed-off-by: Kateryna Nezdolii <nezdolik@spotify.com>

Mirrored from https://github.com/envoyproxy/envoy @ 07e3e287189dc1ce4c54b4e0252e29ae9cf6c19c

envoy/api/v2/cds.proto

@@ -583,6 +583,10 @@ message Cluster {
     // If panic mode is triggered, new hosts are still eligible for traffic; they simply do not
     // contribute to the calculation when deciding whether panic mode is enabled or not.
     bool ignore_new_hosts_until_first_hc = 5;
+
+    // If set to `true`, the cluster manager will drain all existing
+    // connections to upstream hosts whenever hosts are added or removed from the cluster.
+    bool close_connections_on_host_set_change = 6;
   }
 
   // Common configuration for all load balancer implementations.