Tracking issue: evaluating decoding library

# Summary

During FDS sync with FilOz, we've been made aware of the ask to provide an actors decoding library. We _might_ already have some code in place to facilitate this, but it's not exactly clear how well are we positioned to do this, i.e., how much code reuse are we talking about (perhaps not much).

# Completion Criteria

- [x] Check https://github.com/filecoin-project/builtin-actors/issues/1706 and understand the requirements
- [x] Verify Forest capabilities there (mostly around our `StateDecodeParams` implementation, though `fil-actor-states` might also come in handy).
- [x] Discuss the findings with the team
- [ ] Follow up in https://github.com/filecoin-project/builtin-actors/issues/1706

# Additional Links & Resources