Provide user-implementable serialization and deserialization handlers

[davidmrdavid]

While most JavaScript objects can be encoded to and from JSON with ease, there are still some cases where users might prefer to have the option to implement a serialization handler for our SDK to call internally. The alternative is to recommend that users pass datatypes as strings and manually handle serialization themselves, which isn't particularly elegant.

Since Python already does this, I think there's good reason to consider porting that infrastructure to JS as well.