gdb/python: implement support for sending custom MI async notifications

This commit adds a new Python function, gdb.notify_mi, that can be used
to emit custom async notification to MI channel.  This can be used, among
other things, to implement notifications about events MI does not support,
such as remote connection closed or register change.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Andrew Burgess <aburgess@redhat.com>

gdb/python/python-internal.h

@@ -499,6 +499,11 @@ extern PyObject *gdbpy_execute_mi_command (PyObject *self, PyObject *args,
 
 extern void serialize_mi_results (PyObject *results);
 
+/* Implementation of the gdb.notify_mi function.  */
+
+extern PyObject *gdbpy_notify_mi (PyObject *self, PyObject *args,
+				  PyObject *kw);
+
 /* Convert Python object OBJ to a program_space pointer.  OBJ must be a
    gdb.Progspace reference.  Return nullptr if the gdb.Progspace is not
    valid (see gdb.Progspace.is_valid), otherwise return the program_space