Add withdrawal output type about fuel-specs HOT 1 CLOSED

Uniquely identifying deposits is defined here https://github.com/FuelLabs/fuel-specs/blob/afcb701935f975a327353bd4721f7e0218807ae8/specs/protocol/identifiers.md#utxo-id (pending actually having a deposit format, which we can defer until the smart contract implementation begins), as of #95 merged yesterday.

keeping them the same as before unless further changes are required

I disagree. There's no benefit to restricting ourselves to anything in the v1.x design, which was made ad-hoc without fully planning things out. The smart contracts will be re-written and changing the deposit format is trivial in a properly-abstracted codebase.

I would keep the same properties for deposit, namely the block number, amount, token and receiver.

This doesn't match up with the documented deposit ID (https://github.com/FuelLabs/docs/blob/e2d57505997a9a8af0438d57191df2ab024af04d/src/v1.0.0/3.%20Concepts/1.%20Data%20Structures/Deposits.md#deposit):

Deposits are uniquely keyed by { owner, token, blockNumber }

from fuel-specs.