cds: add ring_hash_lb config (#202)

Signed-off-by: Daniel Hochman <danielhochman@users.noreply.github.com>

api/cds.proto

@@ -272,4 +272,29 @@ message Cluster {
     repeated LbSubsetSelector subset_selectors = 3;
   }
   LbSubsetConfig lb_subset_config = 22;
+  
+  message RingHashLbConfig {
+    // Minimum hash ring size, i.e. total virtual nodes. A larger size
+    // will provide better request distribution since each host in the
+    // cluster will have more virtual nodes. Defaults to 1024. In the case
+    // that total number of hosts is greater than the minimum, each host will
+    // be allocated a single virtual node.
+    google.protobuf.UInt64Value minimum_ring_size = 1;
+
+    message DeprecatedV1 {
+      // Envoy uses xxHash by default in v2. Previously std::hash was used
+      // which can vary based on platform. This field exists for migration
+      // purposes.
+      google.protobuf.BoolValue use_std_hash = 1;
+    }
+    DeprecatedV1 deprecated_v1 = 2;
+  }
+
+  // Optional configuration for the load balancing algorithm selected by
+  // LbPolicy. Currently only RING_HASH has additional configuration options.
+  // Specifying ring_hash_lb_config without setting the LbPolicy to RING_HASH
+  // will generate an error at runtime.
+  oneof lb_config {
+    RingHashLbConfig ring_hash_lb_config = 23;
+  }
 }