Write the skeleton for nv27. This is part of a series of tickets across multiple repos, as building up a network upgrade skeleton requires changes across:

• Skeleton for nv27 to support development and testing ref-fvm#2174
• Skeleton for nv27 to support development and testing filecoin-ffi#530
• Skeleton for nv27 to support development and testing go-state-types#383
• Lotus (this ticket)

How to create a skeleton in Lotus is documented here: https://github.com/filecoin-project/lotus/blob/master/documentation/misc/Building_a_network_skeleton.md#lotus-checklist

Done Criteria

• There is a network skeleton in Lotus, which bubbles up all the other dependencies, and allows one to run a 2k-network and see that it switches network version from nv26 --> nv27