api,introspection: new `Data' property in the SMS interface

We need to expose the raw data for the case where we get SMS messages with
binary content (e.g. settings SMS).

introspection/org.freedesktop.ModemManager1.Sms.xml

@@ -58,6 +58,16 @@
     -->
     <property name="Text" type="s" access="readwrite" />
 
+    <!--
+        Data:
+
+        Message data.
+
+        When sending, if the data is larger than the limit of the technology or
+        modem, the message will be broken into multiple parts or messages.
+    -->
+    <property name="Data" type="ay" access="readwrite" />
+
     <!--
         SMSC: