Allow passing in custom `Converter` instances

[cschleiden]

Currently all workflow/activity arguments and results are marshaled using encoding/json. It should be possible to use custom converters to, for example, support binary formats.

[zaataylor]

This is an interesting idea I also stumbled upon while looking at some of the original converter code! Specifically, I was thinking about a Protobuf serialization use case and wondering how I might use something like encoding/protojson instead of the default encoding/json.