This PR adds two commands to l2geth:

- `geth init <url> <genesisHash>`, which will download a remote genesis JSON file, compare its hash to the one specified on the CLI, then reinitialize the chain config.
- `geth dump-chain-cfg`, which will dump the current chain config to stdout.

These commands will make it easier for us to adopt the upcoming gas schedule hardfork.

Fixes: ENG-1906