Message/VRF Signing wiring

Use: https://github.com/filecoin-project/lotus/blob/0c7037b56f901f66008c36509abdfea6701a7d3d/node/impl/full/wallet.go#L39-L47

It needs a Domain Separation Tag that is non-conflicting with ones for messages and other VRF use cases; there is one in F3 right now, but it is quite short.