Start sync on FCU

[MariusVanDerWijden]

Rationale

Right now we need to wait until the beacon chain sends us a new payload followed by an FCU until we start syncing. We could (in theory) just fetch the payload that we are FCU'd to from the network and start syncing way earlier.
This is a big problem on networks without new blocks being built, since we will not be able to sync them without new fork-choices

[etan-status]

Thaaanks :-)

That would also make efforts such as ethereum/execution-apis#318 unnecessary.

[etan-status]

Likewise, light CL nodes could be simplified a lot if only fcU is required to sync Geth: https://github.com/status-im/nimbus-eth2/blob/c668800479296e8cebb9f79be6b7be51512911eb/beacon_chain/nimbus_light_client.nim#L212-L221

Light CL nodes would no longer be required to leech beacon blocks gossip (they cannot fully validate, so can't be active peers on it) to drive EL while the full CL is undergoing maintenance.

Great feature if you can make it happen 🙏

[arnetheduck]

Conversely, it might useful to also start sync on newPayload to fetch side chains as part of exploring their validity, for example after an execution client bug :) this would allow the CL to discard such chains after optimistically having synced them without choosing them as the current fork.