Lazy registration for gob encoding

[maxdml]

This PR does two things:

• Move from registering types with gob during workflow registration / RunAsStep. This solves the case where the workflow or step types is an interface. We cannot identify the underlying concrete type and this will always panic.
• Skip encoding if the value is a nil pointer wrapped in an interface (gob loses the ability to handle it properly.)

Example panic for the 2nd case:

failed to encode data: gob: gob: cannot encode nil pointer of type *dbos.PointerResultStruct inside interface

[kraftp]

How does deserialization during recovery work if types are only registered at serialization time?

[maxdml]

How does deserialization during recovery work if types are only registered at serialization time?

Good point, it doesn't and we need to keep the lazy registration from RunAsStep. During recovery:

• If a recovered workflow is already done, nothing will happen: the input/output are not read during RunWorkflow, and the recovery process dismisses the polling handle it obtains from RunWorkflow
• If a recovered workflow is not done, it'll re-execute all the steps, decoding the output of completed ones from the DB. That's where we need the RunAsStep lazy registration.

And to re-iterate the generalized challenge:

If an application reads any workflow/step input/output where the types have not been registered by the current code (e.g., previous version or recover a workflow which does ListWorkflows for other workflows that have not been seen by the runtime yet.), the read will error because we can't decode

Another challenge btw is being able to test all these edge cases. The gob registry is private, so not exactly easy to wipe and simulate, say, a fresh process recovery scenario.

[maxdml]

Given that it'll be very complicated, if not outright impossible, to understand yet-unseen user defined types, I think we should:

• Factor out the serializer as an interface that be provided by the user
• Use JSON as a default serializer. It'll work for most data.
• Also provide a gob serializer that will do lazy registration (and we can do some more for the user) and document clearly what types must be registered.