Add support for broadcast push notifications

[jchambers]

This is (for now!) a work-in-progress draft of support for broadcast push notifications. It turns out it's a pretty big project that involves building a client for an entirely new part of APNs: the channel management service.

When it's done, this will close #1081.

TODO:

• Channel management system (Add a broadcast channel management client #1107)
  • Tests
  • Docs
• Plumbing for actually sending broadcast notifications
  • Notification/response interfaces
  • Actual implementation
  • Tests
  • Docs
• New payload fields
• Update metrics interfaces
• Update copyright headers

[jchambers]

For folks following along: it turns out this is an enormous project. It's certainly not a matter of just adding a couple new fields to payloads. There's an entire new API for managing broadcast channels, and there's an entirely new endpoint for sending broadcast notifications—which are distinct from normal push notifications.

I'll almost certainly subdivide this into multiple pull requests, but we're looking at several thousand lines of changes before this is done.