IOS POC webpush with vapid

[Arsnael]

After linagora/james-project#5059 is implemented, do a POC to show that we can use webpush with vapid on ios apps instead of using the firebase custom code.

[chibenwa]

For mobile POCs maybe it is easier with the Flutter dudes?

+1 to have a "from scratch POC" that could act as a feasability demonstrator

[Arsnael]

I added this to the tmail frontend board (I think?)