[Feature Request][Firebase Database] Parse Date object automatically

## What feature would you like to see?

Currently, if we have a `Date` object in our data class:
```kotlin
data class Message(
val id: String = "",
val sentAt: Date = Date(),
val sentBy: String = "",
val text: String = "",
)
```

Then use `getValue`:
```kotlin
val message = snapshot.getValue(Message::class.java)
```

It will throw an exception:
```
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.iexample.app, PID: 9218
com.google.firebase.database.DatabaseException: Can't convert object of type java.lang.Long to type java.util.Date
```

This does not happen in Firestore, the object mapper can map a `Timestamp` to a `Date` without any problem.
I get that in Firebase Database a `Date` is stored as a `Long`, but we can convert it easily like this `Date(timeInMillis)`. I hope that this conversion is implemented in the mapper so that we don't need to do any workaround or convert it manually.

## How would you use it?

Usage doesn't change of course.