Modularize the `op-node` command for creating L2 genesis
blocks such that it can read a starting L1 block from disk
instead of needing to fetch it from a node. This makes the
process more simple and reproducible, because the block
JSON file can be written to disk and committed into a repo.

Also add the script for calling the `op-node` L2 genesis generation
to `contracts-bedrock`. It will create the L2 genesis state
in `contracts-bedrock/.testdata` which will be read by
`vm.readAllocs(string)` to populate the initial state.