The `eth1_monitor` check to require engine API from bellatrix onward
has issues in setups where the EL and CL are started simultaneously
because the EL may not be ready to answer requests by the time that the
check is performed. This can be observed, e.g., on Raspberry Pi 4 when
using Besu as the EL client. Now that the merge transition happened, the
check is also not that useful anymore, as users have other ways to know
that their setup is not working correctly (e.g., repeated exchange logs)
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.
