feat: restful -> grpc gateway (#2155)

* Revert "chore: remove unimplemented gateway (#2139)"

This reverts commit d70e73ec66.

* feat: working gateway

* feat: use mr to make it faster

* feat: working gateway

* chore: add comments

* feat: support protoset besides reflection

* feat: support zrpc client conf

* docs: update readme

* feat: support grpc-metadata- header to gateway- header conversion

* chore: add docs

gateway/readme.md

@@ -0,0 +1,56 @@
+# Gateway
+
+## Usage
+
+- main.go
+
+```go
+var configFile = flag.String("f", "config.yaml", "config file")
+
+func main() {
+	flag.Parse()
+
+	var c gateway.GatewayConf
+	conf.MustLoad(*configFile, &c)
+	gw := gateway.MustNewServer(c)
+	defer gw.Stop()
+	gw.Start()
+}
+```
+
+- config.yaml
+
+```yaml
+Name: demo-gateway
+Host: localhost
+Port: 8888
+Upstreams:
+  - Grpc:
+      Etcd:
+        Hosts:
+        - localhost:2379
+        Key: hello.rpc
+    # protoset mode
+    ProtoSet: hello.pb
+    Mapping:
+      - Method: get
+        Path: /pingHello/:ping
+        Rpc: hello.Hello/Ping
+  - Grpc:
+      Endpoints:
+        - localhost:8081
+    # reflection mode, no ProtoSet settings
+    Mapping:
+      - Method: post
+        Path: /pingWorld
+        Rpc: world.World/Ping
+```
+
+## Generate ProtoSet files
+
+- example command
+
+```shell
+protoc --descriptor_set_out=hello.pb hello.proto
+```
+