Add new transaction type variant: "Mint"

[Voxelot]

Rather than managing block rewards (coinbase) implicitly via magic hidden state changes or via block-level coin outputs, we've opted to make a special transaction variant that allows for the creation of assets in special circumstances. The advantage of using a special transaction type for fees, is that it requires virtually no changes to how we manage blockchain state or allow PoA nodes to withdraw fees. It also plays nice with fraud proofs later on.

In addition to Script and Create transaction types, we need to also add a Mint variant as described in the specs: FuelLabs/fuel-specs#406