gsocketconnectable: Add a to_string() virtual method

Add string serialisation functions for GNetworkAddress, GSocketAddress,
GUnixSocketAddress, GInetSocketAddress, GNetworkService and
GSocketConnectable. These are intended for use in debug output, not for
serialisation in network or disc protocols.

They are implemented as a new virtual method on GSocketConnectable:
g_socket_connectable_to_string().

GInetSocketAddress and GUnixSocketAddress now implement
GSocketConnectable directly to implement to_string(). Previously they
implemented it via their abstract parent class, GSocketAddress.

https://bugzilla.gnome.org/show_bug.cgi?id=737116

gio/gsocketconnectable.c

@@ -146,3 +146,34 @@ g_socket_connectable_proxy_enumerate (GSocketConnectable *connectable)
   else
     return (* iface->enumerate) (connectable);
 }
+
+/**
+ * g_socket_connectable_to_string:
+ * @connectable: a #GSocketConnectable
+ *
+ * Format a #GSocketConnectable as a string. This is a human-readable format for
+ * use in debugging output, and is not a stable serialization format. It is not
+ * suitable for use in user interfaces as it exposes too much information for a
+ * user.
+ *
+ * If the #GSocketConnectable implementation does not support string formatting,
+ * the implementation’s type name will be returned as a fallback.
+ *
+ * Returns: (transfer full): the formatted string
+ *
+ * Since: 2.48.0
+ */
+gchar *
+g_socket_connectable_to_string (GSocketConnectable *connectable)
+{
+  GSocketConnectableIface *iface;
+
+  g_return_val_if_fail (G_IS_SOCKET_CONNECTABLE (connectable), NULL);
+
+  iface = G_SOCKET_CONNECTABLE_GET_IFACE (connectable);
+
+  if (iface->to_string != NULL)
+    return iface->to_string (connectable);
+  else
+    return g_strdup (G_OBJECT_TYPE_NAME (connectable));
+}