Allow /validator apis to work pre-genesis

[pawanjay176]

Issue Addressed

N/A

Proposed Changes

Lighthouse BN http endpoint would return a server error pre-genesis on the validator/duties/attester and validator/prepare_beacon_proposer because slot_clock.now() would return a None pre-genesis.

The prysm VC depends on the endpoints pre-genesis and was having issues interoping with the lighthouse bn because of this reason.

The proposer duties endpoint explicitly handles the pre-genesis case here

lighthouse/beacon_node/http_api/src/proposer_duties.rs

Lines 23 to 28 in 538067f

	let current_epoch = chain
	.slot_clock
	.now_or_genesis()
	.map(|slot| slot.epoch(T::EthSpec::slots_per_epoch()))
	.ok_or(BeaconChainError::UnableToReadSlot)
	.map_err(warp_utils::reject::unhandled_error)?;

I see no reason why we can't make the other endpoints more flexible to work pre-genesis. This PR handles the pre-genesis case on the attester and prepare_beacon_proposer endpoints as well.

Thanks for raising @james-prysm.

[barnabasbusa]

Can confirm, prysm vc now works with ethpandaops/lighthouse:pawanjay176-duties-pre-genesis-552421c

participants:
  - el_type: nethermind
    cl_type: lighthouse
    cl_image: ethpandaops/lighthouse:pawanjay176-duties-pre-genesis-552421c
    vc_type: prysm
    count: 2
additional_services:
  - dora