Add support for exporting the EL chain configuration.

The OP Stack now requires an L1 chain configuration file to run the node ([source](https://github.com/ethereum-optimism/optimism/blob/aca9c3f010d81fb22829b01a65b8c9e22544e1fd/op-node/service.go#L330)). It would be great if the `ethereum-package` could provide a way to export or extract this chain config file.