ref. https://pkg.go.dev/github.com/ava-labs/avalanchego/vms/platformvm#Tx
ref. https://pkg.go.dev/github.com/ava-labs/avalanchego/vms/platformvm#UnsignedAddSubnetValidatorTx
ref. https://pkg.go.dev/github.com/ava-labs/avalanchego/vms/platformvm#UnsignedTx