[Tunnelbroker] implement creating APNs headers

Summary:
Implementing building request headers.

Apple docs: Send a POST request to APNs.

Not sure about PushType - perhaps we can just use String but I prefer more detailed type definition. (cc. @marcin).

Depends on D12611

Test Plan: Tested calling this function with different params

Reviewers: bartek, marcin

Reviewed By: bartek, marcin

Subscribers: ashoat, tomek, marcin

Differential Revision: https://phab.comm.dev/D12612