feat: 基本广播服务

- 由Hub接收/push/{topic}的请求并解析信息体广播到对应的Client

interval/protocol/types.go

@@ -0,0 +1,17 @@
+package protocol
+
+type MessageType string
+
+const (
+	MsgInit        MessageType = "init"
+	MsgSubscribe   MessageType = "subscribe"
+	MsgUnsubscribe MessageType = "unsubscribe"
+	MsgBroadcast   MessageType = "broadcast"
+	MsgError       MessageType = "error"
+)
+
+type Topic string
+
+func (t Topic) Valid() bool {
+	return t != ""
+}