Provide FCM as a connector

[pimeys]

I have an ongoing project to create a unified push client for fcm, apns2 and web-push. Soon hyper will get h2 support, so I can finally finish my long-lasting project to clean the rust-apns2 and make it async+fast. For apns2 I first took the old client as basis, but decided to rewrite it from scratch.

My refactoring there has now connectors which handle the connection to the push service, including authentication, and if we'd go that route with FCM and web-push, then I could use all these libraries to create a master lib to connect all the services.

Shouldn't be lots of work, but would this be useful?

https://github.com/carllerche/h2
https://github.com/pimeys/rust-web-push
https://github.com/polyitan/apns2