Support Mint for coinbase

[Voxelot]

related to: FuelLabs/fuel-specs#286, FuelLabs/fuel-tx#193, FuelLabs/fuel-vm#225

The block producer needs to include a single Mint transaction in each block to capture the fees of the block, with the owner set to a configured wallet address (does not need to match the PoA consensus key).

When executing a block in production mode, the executor will set the correct output amount of the mint transaction to match the coinbase award amount of the block.

When executing a block in validation mode, the executor will verify that:

• No more than 1 mint transaction exists in the block
• The mint only includes a single output
• The single mint output is of AssetId::BASE
• The amount of the mint output is equivalent to the expected coinbase amount from the gas fees

The coinbase mint transaction does not incur any gas fees itself.