Support MDM payload

The push notification for mobile device manager (MDM) checkin messages is different from standard APNS messages.  It would be GREAT if this support was added to apns-http2.  Pushy and other libs support it.  More information is available here:

https://developer.apple.com/business/documentation/MDM-Protocol-Reference.pdf#//apple_ref/doc/uid/TP40017387-CH3-SW2
