Add apns-push-type header

[gferon]

Set the apns-push-type header (mostly to support background notifications, but I mapped all the possible values).

Due Dilligence

• [ ] Breaking change
• [x] Requires a documentation update
• [ ] Requires a e2e/integration test update

[r10s]

great to find this PR, very nice! seems we're requiring exactly that for https://github.com/deltachat/notifiers/issues/22

[gibix]

Yep, just tested, works as expected

[r10s]

we're using this PR also for https://github.com/deltachat/notifiers in practise meanwhile, works like a charm!

is there sth. we can help that this PR gets merged?