Provide user-implementable serialization and deserialization handlers

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.

Start by locating the JavaScript SDK's existing JSON serialization and deserialization paths, then compare them with the serialization infrastructure already present in Python. Define the user-facing handler API and its integration points before implementing it, and verify that supported custom datatypes can round-trip through the SDK.

Written by the indexing model from the issue text.