Support for GObject instance_init in Glib.Object.Initialize_Class_Record

Description

Currently Glib.Object.Initialize_Class_Record does not allow registering an instance_init function.
In the implementation (src/misc.c) the field is hardcoded to NULL:

info.instance_init = NULL;

This makes it impossible to create composite custom widgets that are properly initialized when instantiated by Gtk.Builder (or g_object_new).

In C, the standard way to initialize the children of a composite widget is the instance_init function. Without it, Ada developers are forced to call Initialize manually after every Builder.Get_Object, which is error-prone and inconsistent with how native widgets work.

Proposed solution
Extend Initialize_Class_Record (by add an overload / new parameter) so that the user can optionally provide an Instance_Init callback. This callback should be stored in the AdaGObjectClassRecord and assigned to info.instance_init when registering the type.

This would bring GtkAda closer to normal GObject practices and improve the experience of writing reusable composite widgets that work with Glade/GtkBuilder.

Additional context
The current limitation affects anyone writing composite widgets in Ada that need to work with UI files loaded by GtkBuilder.

Start in src/misc.c at Glib.Object.Initialize_Class_Record and inspect how AdaGObjectClassRecord is populated, especially the hardcoded info.instance_init = NULL. Trace the related declaration and registration path, then verify that an optional Instance_Init callback reaches info.instance_init and supports composite widgets created through Gtk.Builder or g_object_new.