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