base-modem: define new helper MMBaseModemAtCommandAlloc

It has the same exact format as MMBaseModemAtCommand, but its contents
are assumed heap allocated.

The only real purpose of this type is to allow defining static
constant MMBaseModemAtCommand variables without warnings when using
-Wdiscarded-qualifiers.

plugins/sierra/mm-broadband-modem-sierra.c

@@ -1805,8 +1805,8 @@ parse_time_reply (MMBaseModem *self,
 }
 
 static const MMBaseModemAtCommand time_check_sequence[] = {
-    { (gchar *) "!TIME?", 3, FALSE, parse_time_reply },    /* 3GPP */
-    { (gchar *) "!SYSTIME?", 3, FALSE, parse_time_reply }, /* CDMA */
+    { "!TIME?", 3, FALSE, parse_time_reply },    /* 3GPP */
+    { "!SYSTIME?", 3, FALSE, parse_time_reply }, /* CDMA */
     { NULL }
 };