Draft: iOS Mail app push notification support

This isn't ready for merging, but I don't know if I'll have the time to polish it, so I'll just offer what I have. This enables push notifications on mail receipt in the iOS default Mail app. Apple's push infrastructure is used to deliver notification from the mail server to the iOS device with the Mail app, which notifies the user according to the on-device settings.

Variants of this configuration have been working for me for almost 3 years up to iOS 16.3.1. (I should note that people using this feature are nervous Apple will shut it down, since they discontinued the Mac OS X Server product it was created for. On the other hand, Fastmail supports it, so maybe they have some sway with Apple to keep it alive.)

What needs to be done:

• add "enable" flag instead of assuming enable
• make the Apple ID email and password configurable - they are coded into the Nix source itself right now.
  • This is for the provisioning of Apple push certificates and doesn't need to have any relation to your hosted email accounts.
  • You should use the Apple ID you've purchased macOS Server with.
  • There is no support for an app password if you have 2FA. Use your normal password. No other special type or configuration of the account is required.
• add some documentation, such as explaining the above in the config definitions. The xaps repositories may have some pointers on troubleshooting, some more technical details about how it works, and privacy implications. I can probably provide those if needed too, but I don't remember the details off hand.