Support the FCM HTTP V1

[msmykowski]

Pigeon currently uses the legacy FCM. We would like to upgrade to the new FCM api, but Pigeon currently does not support it.

https://firebase.google.com/docs/cloud-messaging/migrate-v1

[hpopp]

This has been on my roadmap for a while, likely for a Pigeon v2 release. I need to properly plan through API changes and see if there's any potential issues.

[tmpduarte]

@hpopp are there any ETA for this? Is there anything I can help with (I will only have time after mid september 😢 )?

[hpopp]

Heh, I've been absolutely swamped with deadlines over the past few weeks. My plan is to pick it back up around mid september again!

[hpopp]

Support for FCM v1 has now been merged to master. The only thing left is testing and polish. Any feedback on design choices would be much appreciated.