fix genesis time calc

[ltitanb]

For custom chains, we calculate the genesis time as min_genesis_time + genesis_delay, this seems to hold for most testnets, but can be different than the actual genesis time (e.g. it's different in mainnet).

commit-boost-client/crates/common/src/types.rs

Lines 236 to 239 in f38d309

	// note that this can be wrong, (e.g. it's wrong in mainnet). The correct
	// value should come from /eth/v1/beacon/genesis
	// more info here: https://kb.beaconcha.in/ethereum-staking/the-genesis-event
	// FIXME