dubbo_proxy: implement dubbo filter (#4896)

*Description*:  Add dubbo proxy filter
*Risk Level*: low
*Testing*: unittest, manual integration test
*Docs Changes*: N/A
*Release Notes*: N/A
[Optional Fixes #Issue]  #3998  #4200

Signed-off-by: tianqian.zyf <tianqian.zyf@alibaba-inc.com>

Mirrored from https://github.com/envoyproxy/envoy @ 7f1bbfaceb44c51e0c7734c7c0abe0afa00f39f6

envoy/config/filter/network/dubbo_proxy/v2alpha1/BUILD

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

envoy/config/filter/network/dubbo_proxy/v2alpha1/README.md

@@ -0,0 +1 @@
+Protocol buffer definitions for the Dubbo proxy.

envoy/config/filter/network/dubbo_proxy/v2alpha1/dubbo_proxy.proto

@@ -0,0 +1,25 @@
+syntax = "proto3";
+
+package envoy.extensions.filters.network.dubbo_proxy.v2alpha1;
+option go_package = "v2";
+
+import "validate/validate.proto";
+
+// [#protodoc-title: Extensions Thrift Proxy]
+// Thrift Proxy filter configuration.
+message DubboProxy {
+  // The human readable prefix to use when emitting statistics.
+  string stat_prefix = 1 [(validate.rules).string.min_bytes = 1];
+
+  // Configure the protocol used.
+  enum ProtocolType {
+    Dubbo = 0; // the default protocol.
+  }
+  ProtocolType protocol_type = 2 [(validate.rules).enum.defined_only = true];
+
+  // Configure the serialization protocol used.
+  enum SerializationType {
+    Hessian2 = 0; // the default serialization protocol.
+  }
+  SerializationType serialization_type = 3 [(validate.rules).enum.defined_only = true];
+}