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/headerbuilder.go

@@ -0,0 +1,29 @@
+package gateway
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+)
+
+const (
+	metadataHeaderPrefix = "Grpc-Metadata-"
+	metadataPrefix       = "gateway-"
+)
+
+func buildHeaders(header http.Header) []string {
+	var headers []string
+
+	for k, v := range header {
+		if !strings.HasPrefix(k, metadataHeaderPrefix) {
+			continue
+		}
+
+		key := fmt.Sprintf("%s%s", metadataPrefix, strings.TrimPrefix(k, metadataHeaderPrefix))
+		for _, vv := range v {
+			headers = append(headers, key+":"+vv)
+		}
+	}
+
+	return headers
+}