Add new official supported types #191

Thanks for stopping by to let us know something could be better!

Is your feature request related to a problem? Please describe.
Yes, we want to map BigDecimal or LocalDateTime object in our Firestore database.
For now, we have these problems :

java.lang.IllegalArgumentException: Could not serialize object. Numbers of type BigDecimal are not supported, please use an int, long, float or double (found in field 'insurances.liability.amount.value')
	at com.google.cloud.firestore.CustomClassMapper.serializeError(CustomClassMapper.java:555) ~[google-cloud-firestore-1.32.4.jar:1.32.4]

Describe the solution you'd like
I would like official mappers for conventionnal Java types.

Describe alternatives you've considered
Create our custom (De)Serializer but I think that it can be a good thing for the lib.

Additional context