Add support for custom networks via JSON config file

[merklefruit]

op-node has a flag --rollup.config=/rollup.json to allow it to use network configuration derived from a JSON file.
This would be a useful addition to Magi, for the following reasons:

1. Right now there is no way to use a different network than the two pre-configured ones
2. This would make it a lot easier to run Magi on a devnet or any custom op-stack network