Add an implmenetation of Notification for MinGW

Add an alternative implementation of the Notification class for MinGW
using a Windows manual-reset event object.

GCC version < 13 in MinGW, if configured with the win32 thread model
(with the --enable-threads=win32 configure option), does not implement
std::mutex and std::condition_variable in the <mutex> and
<condition_variable> headers. So the current implementation of the
Notification class, which uses std::mutex and std::condition_variable,
has compilation errors. Windows has a synchronization object called a
manual-reset event object that behaves just like the Notification class.
So we can easily implement the Notification class using a Windows
manual-reset event object.

This GCC issue is fixed in GCC 13. See
https://gcc.gnu.org/gcc-13/changes.html#windows. This alternative
implementation of Notification should be removed when GoogleTest
requires GCC version >= 13 in MinGW.

Fixes https://github.com/google/googletest/issues/4031 and
https://github.com/google/googletest/issues/4464.

PiperOrigin-RevId: 888752598
Change-Id: I69e01b2e7953e1fe72d87b6d0804c7a8a8d0e740