udp_proxy: scaffolding (#8883)

This is the first commit in a series to support UDP proxying.
There are quite a few TODOs in the code before this feature
will be considered a MVP.

Part of https://github.com/envoyproxy/envoy/issues/492

Mirrored from https://github.com/envoyproxy/envoy @ 477fafdaa8423cff1a5c22d58904c22eed9155f3

envoy/config/filter/network/tcp_proxy/v2/tcp_proxy.proto

@@ -113,7 +113,6 @@ message TcpProxy {
     option (validate.required) = true;
 
     // The upstream cluster to connect to.
-    //
     string cluster = 2;
 
     // Multiple upstream clusters can be specified for a given route. The

envoy/config/filter/network/tcp_proxy/v3alpha/tcp_proxy.proto

@@ -61,7 +61,6 @@ message TcpProxy {
     option (validate.required) = true;
 
     // The upstream cluster to connect to.
-    //
     string cluster = 2;
 
     // Multiple upstream clusters can be specified for a given route. The

envoy/config/filter/udp/udp_proxy/v2alpha/BUILD

@@ -0,0 +1,7 @@
+# DO NOT EDIT. This file is generated by tools/proto_sync.py.
+
+load("@envoy_api//bazel:api_build_system.bzl", "api_proto_package")
+
+licenses(["notice"])  # Apache 2
+
+api_proto_package()

envoy/config/filter/udp/udp_proxy/v2alpha/udp_proxy.proto

@@ -0,0 +1,22 @@
+syntax = "proto3";
+
+package envoy.config.filter.udp.udp_proxy.v2alpha;
+
+option java_outer_classname = "UdpProxyProto";
+option java_multiple_files = true;
+option java_package = "io.envoyproxy.envoy.config.filter.udp.udp_proxy.v2alpha";
+
+import "google/protobuf/duration.proto";
+
+import "validate/validate.proto";
+
+// TODO(mattklein123): docs
+
+message UdpProxyConfig {
+  oneof cluster_specifier {
+    option (validate.required) = true;
+
+    // The upstream cluster to connect to.
+    string cluster = 1 [(validate.rules).string = {min_bytes: 1}];
+  }
+}