feat: added XAPPLEPUSHSERVICE support (per #711)

[titanism]

This adds command support for XAPPLEPUSHSERVICE with a commented out IMAP command handler showing an example implementation.

[titanism]

Got it working!!!! 🎉

This PR is ready for merge, please review and merge, thank you! ✅

If you'd like to see the working implementation, see these files:

• Our handler for XAPPLEPUSHSERVICE @ https://github.com/forwardemail/forwardemail.net/blob/master/helpers/imap/on-xapplepushservice.js which simply stores to database the user's APS data provided via this command
• Sending of Apple Push Notification (APN) @ https://github.com/forwardemail/forwardemail.net/blob/master/helpers/send-apn.js

Feel free to try it out on https://forwardemail.net with your iOS device (setup an IMAP account)

PUSH now supported on iOS!!!! ⚡ (our alternative since IDLE not supported on iOS Mail)

[titanism]

OK it should be good to go now, I had one missing line of code (forgot to invoke this.send) with APS version and sub topic.

titanism@6e006db

PR ready.

[andris9]

Thanks for figuring this stuff out. I still need to edit it a bit; the commented-out code is not ideal; I will probably add a configuration option or smth.

[titanism]

Sounds good @andris9! Thank you 🙏

[andris9]

Added the following changes (4ef3eaa):

• fixed CAPABILITY listing for XAPPLEPUSHSERVICE (only visible for authenticated sessions)
• made XAPPLEPUSHSERVICE configurable. Requires aps.enabled option to be true, otherwise will reject the command
• updated schema handling (previously invalid input like tag XAPPLEPUSHSERVICE with no arguments caused an uncaught exception)