feat: Conform SynthetixBridge contracts with Optimism standard bridges

[nicholaspai]

Optimism L1 standard bridge interface: https://github.com/ethereum-optimism/optimism/blob/c258acd495f884f10c3da82510a8f378222f4f92/packages/contracts/contracts/L1/messaging/IL1StandardBridge.sol

Optimism L2 standard bridge interface: https://github.com/ethereum-optimism/optimism/blob/c258acd495f884f10c3da82510a8f378222f4f92/packages/contracts/contracts/L2/messaging/IL2ERC20Bridge.sol

Adding these functions to future bridge interface will allow cross-chain Bridges like Across for arbitrary ERC20's to support tokens like SNX that have custom bridges on Optimism

For example, DAI has a custom Optimism bridge that does implement the same interface: https://github.com/makerdao/optimism-dai-bridge/blob/master/contracts/l2/L2DAITokenBridge.sol#L92