Memory usage/limits with extensions

Several iOS extensions have strict memory limits, and we should figure out what those limits currently are (as far as we can, since they're not documented), and tweak the GC to try to not go over that limit.

References:
* https://github.com/xamarin/xamarin-macios/pull/3734
* https://bugzilla.xamarin.com/show_bug.cgi?id=53207 FileProviderExtension with test case
* https://bugzilla.xamarin.com/show_bug.cgi?id=58167 Notification Service extension
* https://bugzilla.xamarin.com/show_bug.cgi?id=56022 Improve the static registrar to generate code that does not need any reflection at runtime
* https://bugzilla.xamarin.com/show_bug.cgi?id=43985 UNNotificationServiceExtension
* https://bugzilla.xamarin.com/show_bug.cgi?id=60597 FileProviderExtension with test case
* https://forums.developer.apple.com/thread/73148 memory limits for network extensions