OP Stack Custom Gas Token Support

[tynes]

The OP Stack is getting a feature that enables it to use a custom gas token. The spec can be found [here]. We have a guide for chain operators that can be found here on how to deploy a new chain using custom gas token.

The big difference is in the deposit logic itself. A standard approve/transferFrom flow is used to deposit an ERC20 token into L2 where a native asset is minted. This allows the chain to use this custom gas token as if it was ether. It would be amazing if there was support for this in viem.

The scope of work:

• Support for calling depositERC20Transaction (new function on OptimismPortal)
• Support for calling the IGasToken interface on the L1Block contract and the SystemConfig contract to know if a chain is using a custom gas token and which token is the custom gas token
• Support for calling approve on the L1 ERC20 token that is used as the custom gas token so that it can easily be deposited into L2