Adds the `--web3-url` launch argument to `nimbus_light_client` to enable
driving the EL with the optimistic head obtained from LC sync protocol.
This will keep issuing `newPayload` / `forkChoiceUpdated` requests for
new blocks, marking them as optimistic. `ZERO_HASH` is reported as the
finalized block for now.
* Add support for launching Geth and Nimbus-eth1 nodes in launch_local_simulation
* Prevent undefined variables errors when Nimbus and Geth are not launched
Co-authored-by: Nikolay Mitev <github@hmel.org>