dubbo_proxy: Implement the routing of Dubbo requests (#5973)

*Description*: Implement the routing of Dubbo requests
*Risk Level*: low
*Testing*: unit test
*Docs Changes*: N/A
*Release Notes*: N/A
[Optional Fixes #Issue] #3998
[Optional *Deprecated*:]

this is only part of the code for DubboProxy, the rest of the code hasn't been committed yet, complete implementation: https://github.com/gengleilei/envoy/tree/feature-dubbo-router-develop/source/extensions/filters/network/dubbo_proxy

Signed-off-by: leilei.gll <leilei.gll@alibaba-inc.com>

Mirrored from https://github.com/envoyproxy/envoy @ 768fde9abb502b000f2c104f82a9501bed3c28a5

envoy/config/filter/dubbo/router/v2alpha1/BUILD

@@ -0,0 +1,8 @@
+load("//bazel:api_build_system.bzl", "api_proto_library_internal")
+
+licenses(["notice"])  # Apache 2
+
+api_proto_library_internal(
+    name = "router",
+    srcs = ["router.proto"],
+)

envoy/config/filter/dubbo/router/v2alpha1/router.proto

@@ -0,0 +1,14 @@
+syntax = "proto3";
+
+package envoy.config.filter.dubbo.router.v2alpha1;
+
+option java_outer_classname = "RouterProto";
+option java_multiple_files = true;
+option java_package = "io.envoyproxy.envoy.config.filter.dubbo.router.v2alpha1";
+option go_package = "v2alpha1";
+
+// [#protodoc-title: Router]
+// Dubbo router :ref:`configuration overview <config_dubbo_filters_router>`.
+
+message Router {
+}