externalize enr_fork_id logic

[jrhea]

mothra should not maintain the logic for this. Initially, something like this should work:

        EnrForkId {
            fork_digest: compute_fork_digest([0; 4],[0;32]).to_le_bytes(),
            next_fork_version: [0;4], //genesis_fork_version,
            next_fork_epoch: u64::max_value(), //far_future_epoch,
        }

where compute_fork_digest is define in spec