[WIP] add manage FCM tokens sample

[kroikie]

[puf]

Nice!

I often refer to this code in the FCM sample on how to prune tokens while sending. Is it necessary to use both approaches, or would one or the other suffice?

If both are needed, can you make sure we link them to each other?

[egilmorez]

Not to pre-empt a response from Arthur, but I'd say that we are indeed recommending the use of multiple approaches to maintain token health. The sample you cite Puf -- and thanks, because I think I can use it if we add some start/end tags -- pertains to this approach I think: https://firebase.google.com/docs/cloud-messaging/manage-tokens#detect-invalid-token-responses-from-the-fcm-backend

General comment for Arthur: can you add those tags if appropriate, and also tag the other two functions in your index.js? Thanks!