Added MessageState enumeration and XJB file for message schema

src/main/java/envoy/schema/MessageState.java

@@ -0,0 +1,22 @@
+package envoy.schema;
+
+import javax.xml.bind.annotation.XmlEnum;
+import javax.xml.bind.annotation.XmlType;
+
+/**
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>MessageState.java</strong><br>
+ * Created: <strong>11 Oct 2019</strong><br>
+ * 
+ * @author Kai S. K. Engelbart
+ */
+@XmlType(name = "")
+@XmlEnum
+public enum MessageState {
+
+	Waiting, Sent, Received, Read;
+
+	public String value() { return name(); }
+
+	public static MessageState fromValue(String v) { return valueOf(v); }
+}