Multicast support

[Jasperav]

Thanks a lot for this crate. I want to migrate from the legacy to the HTTP V1 protocol with this crate. However, it is not possible with the crate to use multicasting (https://firebase.google.com/docs/cloud-messaging/send-message#send-messages-to-multiple-devices). Do you think that this crate will support that feature in the foreseeable future?

[sanath-2024]

Hi, thanks for the input! multicast is certainly something that could be supported in the future. The goal is to eventually support the entire FCM API.

[Jasperav]

@sanath-2024 Is this something we can look into next week after your finals? I suspect it isn't that hard, but this is a must-have feature.

[sanath-2024]

@Jasperav for sure, thanks for your patience!