api,simple: new 'multiplex' setting in bearer properties

Both the Simple.Connect() and Modem.CreateBearer() are updated to
allow a new 'multiplex' setting in the properties provided by the user
in both of these methods.

The new setting expects a MMBearerMultiplexSupport enum indicating
what kind of multiplex needs the user has:
 * none: if multiplex must not be used.
 * requested: if multiplex should be used if available.
 * required: if multiplex must be used.

The underlying implementations will take care of accepting or
rejecting the setting depending on the system and modem capabilities.

libmm-glib/mm-common-helpers.c

@@ -857,6 +857,16 @@ mm_common_get_eps_ue_mode_operation_from_string (const gchar  *str,
                               error);
 }
 
+MMBearerMultiplexSupport
+mm_common_get_multiplex_support_from_string (const gchar  *str,
+                                             GError      **error)
+{
+    return _enum_from_string (MM_TYPE_BEARER_MULTIPLEX_SUPPORT,
+                              str,
+                              MM_BEARER_MULTIPLEX_SUPPORT_UNKNOWN,
+                              error);
+}
+
 GArray *
 mm_common_oma_pending_network_initiated_sessions_variant_to_garray (GVariant *variant)
 {