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_test.go

@@ -0,0 +1,21 @@
+package gateway
+
+import (
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBuildHeadersNoValue(t *testing.T) {
+	req := httptest.NewRequest("GET", "/", nil)
+	req.Header.Add("a", "b")
+	assert.Nil(t, buildHeaders(req.Header))
+}
+
+func TestBuildHeadersWithValues(t *testing.T) {
+	req := httptest.NewRequest("GET", "/", nil)
+	req.Header.Add("grpc-metadata-a", "b")
+	req.Header.Add("grpc-metadata-b", "b")
+	assert.EqualValues(t, []string{"gateway-A:b", "gateway-B:b"}, buildHeaders(req.Header))
+}