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

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>