[Remote config] Should complete observables after emitting values

In the `parameter$` helper function you convert the promise to the Observable
As far as the promise emits values only once, we need to complete the observable after getting a value from the promise. We need to add `subscriber.complete();` after [line 31](https://github.com/FirebaseExtended/rxfire/blob/main/remote-config/index.ts#L31)

Or better use `from(ensureInitialized(remoteConfig)).pipe(map(...))`