op-sync-tester: Mock different EL implementations for initial EL Sync

We need to validate L2CL by mimicking multiple L2EL implementations, especially for the EL sync and possibly different behavior around engine API responses.

For initial EL Sync, we may handle different networks(consider regenesis of op-mainnet), related with `l2.enginekind` flag(Introduced at https://github.com/ethereum-optimism/optimism/pull/10767).