Use signal (and why I might not)

[GrandSchtroumpf]

Since Angular 16 can can use Signal to trigger change detection.
I've tried to implement Signal in ngfire, but I think this is not a good API for it :

1 - Signal don't return the amount of listeners

There is currently no way to know if a signal has a listener: angular/angular#50182

2 - Async transformation is not straight foward and might create some weird issues

RXJS provides a simple switchMap async transformation that keep previous value until it changes.
This is not easy to achieve with signals for now : angular/angular#50183