dns: add v4 preferred option (#18108)

Signed-off-by: Jose Nino <jnino@lyft.com>

Mirrored from https://github.com/envoyproxy/envoy @ 6d09a15512e491a577d1d764d270df0ef5deeef2

envoy/config/cluster/v3/cluster.proto

@@ -123,15 +123,23 @@ message Cluster {
   // only perform a lookup for addresses in the IPv6 family. If AUTO is
   // specified, the DNS resolver will first perform a lookup for addresses in
   // the IPv6 family and fallback to a lookup for addresses in the IPv4 family.
+  // This is semantically equivalent to a non-existent V6_PREFERRED option.
+  // AUTO is a legacy name that is more opaque than
+  // necessary and will be deprecated in favor of V6_PREFERRED in a future major version of the API.
+  // If V4_PREFERRED is specified, the DNS resolver will first perform a lookup for addresses in the
+  // IPv4 family and fallback to a lookup for addresses in the IPv6 family. i.e., the callback
+  // target will only get v6 addresses if there were NO v4 addresses to return.
   // For cluster types other than
   // :ref:`STRICT_DNS<envoy_v3_api_enum_value_config.cluster.v3.Cluster.DiscoveryType.STRICT_DNS>` and
   // :ref:`LOGICAL_DNS<envoy_v3_api_enum_value_config.cluster.v3.Cluster.DiscoveryType.LOGICAL_DNS>`,
   // this setting is
   // ignored.
+  // [#next-major-version: deprecate AUTO in favor of a V6_PREFERRED option.]
   enum DnsLookupFamily {
     AUTO = 0;
     V4_ONLY = 1;
     V6_ONLY = 2;
+    V4_PREFERRED = 3;
   }
 
   enum ClusterProtocolSelection {