v3 refactor (#1868)

* Move to v3

Co-authored-by: Ben Toogood <bentoogood@gmail.com>

client/mucp/mucp_message.go

@@ -0,0 +1,40 @@
+package mucp
+
+import (
+	"github.com/micro/go-micro/v3/client"
+)
+
+type message struct {
+	topic       string
+	contentType string
+	payload     interface{}
+}
+
+func newMessage(topic string, payload interface{}, contentType string, opts ...client.MessageOption) client.Message {
+	var options client.MessageOptions
+	for _, o := range opts {
+		o(&options)
+	}
+
+	if len(options.ContentType) > 0 {
+		contentType = options.ContentType
+	}
+
+	return &message{
+		payload:     payload,
+		topic:       topic,
+		contentType: contentType,
+	}
+}
+
+func (m *message) ContentType() string {
+	return m.contentType
+}
+
+func (m *message) Topic() string {
+	return m.topic
+}
+
+func (m *message) Payload() interface{} {
+	return m.payload
+}