Introduce nv21 skeleton for local testing:
- Use local go-state-types with actor_version_checklist changes: https://github.com/filecoin-project/go-state-types/blob/master/actors_version_checklist.md
- Imports mock v12-actors bundle
- Define upgrade heights
- Generate adapters
- Add upgrade schedule and migration
- Add actorstype to the NewActorRegistry in /chain/consensus/computestate.go
- Add upgrade field to api/types.go/ForkUpgradeParams
- Add upgrade to node/impl/full/state.go
- Add network version to chain/state/statetree.go
- make jen
- make docsgen-cli
