encoding/xml: add xml encode support (#905)

* encoding/xml: add xml encode support

* encoding/xml: avoid nil pointer panic

* action: avoid go get golint pollution go.mod

encoding/xml/xml.go

@@ -0,0 +1,37 @@
+package xml
+
+import (
+	"encoding/xml"
+	"reflect"
+
+	"github.com/go-kratos/kratos/v2/encoding"
+)
+
+// Name is the name registered for the xml codec.
+const Name = "xml"
+
+func init() {
+	encoding.RegisterCodec(codec{})
+}
+
+// codec is a Codec implementation with xml.
+type codec struct{}
+
+func (codec) Marshal(v interface{}) ([]byte, error) {
+	return xml.Marshal(v)
+}
+
+func (codec) Unmarshal(data []byte, v interface{}) error {
+	rv := reflect.ValueOf(v)
+	for rv.Kind() == reflect.Ptr {
+		if rv.IsNil() {
+			rv.Set(reflect.New(rv.Type().Elem()))
+		}
+		rv = rv.Elem()
+	}
+	return xml.Unmarshal(data, v)
+}
+
+func (codec) Name() string {
+	return Name
+}