[java-firestore] Allow Optional<T> serialization/deserialization

**Is your feature request related to a problem? Please describe.**

The firestore serializer/deserializer doesn't support Optional\ fields.

**Describe the solution you'd like**

I'd like the firestore serializer/deserializer support reading from Optional\ and parsing responses to Optional\ fields.

**Describe alternatives you've considered**

n/a

**Additional context**

n/a