[WIP] feat: Add nv21 skeleton

[rjan90]

Related Issues

Closes #10993

Proposed Changes

We need to add the skeleton for nv21 without any upgrade epochs, code / state changes / migrations. It should have no effect except on local devnets to unblock testing and tests for SynthPoRep

Additional Info

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, build, chore, ci, docs, perf, refactor, revert, style, test
  • area, e.g. api, chain, state, market, mempool, multisig, networking, paych, proving, sealing, wallet, deps
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green

[rjan90]

Update:

I´m planning to roll the skeleton nv21 work here over to the next sprint to:

• I need some additional time/work to test that the skeleton works - i.e see that network version changes on an upgrade epoch on a local network when re-using v11 actors.
• When actor/fvm code for SynthPoRep is done, get that wired in.

Risks here is that the code is relatively new to me, so takes a bit more time to understand - having some guidance office hours by other devs during sprint would be beneficial to ensure that it is ready as soon as the actor/fvm code is completed

[jennijuju]

(Later can we add a testing plan please ?

[rjan90]

Superseded by #11057