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/config.go

@@ -0,0 +1,35 @@
+package gateway
+
+import (
+	"time"
+
+	"github.com/zeromicro/go-zero/rest"
+	"github.com/zeromicro/go-zero/zrpc"
+)
+
+type (
+	// GatewayConf is the configuration for gateway.
+	GatewayConf struct {
+		rest.RestConf
+		Upstreams []upstream
+		Timeout   time.Duration `json:",default=5s"`
+	}
+
+	// mapping is a mapping between a gateway route and a upstream rpc method.
+	mapping struct {
+		// Method is the HTTP method, like GET, POST, PUT, DELETE.
+		Method string
+		// Path is the HTTP path.
+		Path string
+		// Rpc is the gRPC rpc method, with format of package.service/method
+		Rpc string
+	}
+	// upstream is the configuration for upstream.
+	upstream struct {
+		// Grpc is the target of upstream.
+		Grpc zrpc.RpcClientConf
+		// ProtoSet is the file of proto set, like hello.pb
+		ProtoSet string `json:",optional"`
+		Mapping  []mapping
+	}
+)