Replacing Timestamp with Java8 Instant or ability to use custom serializer #509

This is a follow up for an issue which was closed without any explanation:
#88

Is your feature request related to a problem? Please describe.
We would like to use custom deserializer for using Java8 Instant or similar instead of proprietary Timestamp.

Describe the solution you'd like
The easiest would be registration of custom serializer like Jackson provides:

val objectMapper = ObjectMapper()
    .registerModule(JavaTimeModule())

Additional context
See the previously closed issue.

Note to Googlers (added by @dconeybe): This feature request is being tracked internally by b/326255898