ref-fvm version selector utility

[raulk]

Context

The version policy we agreed on in #358 bumps the major version of ref-fvm whenever ref-fvm changes are needed to accompany a new network version. This means that clients will need to depend and select across several ref-fvm versions to (a) ride network upgrades, and to (b) provide historical chain support. We should make it easy for client implementations to select the right ref-fvm version based on network epochs/NVs.

Proposal

Build a separate crate fvm-selector within this repo that acts a selector and provides a standard, unified interface that can be shimmed to support multiple backing Engines, MachineContexts, Machines, Executors, etc.

More definition needed.