Avoid full Eth2NetworkConfig #267
Description
We load the Eth2NetworkConfig to get the ChainSpec for built in networks, or for parsing it from disk.
This is very wasteful, as there is a lot of other data that we do not need, e.g. the genesis state or KZG setup. We should only load the ChainSpec. Currently, this is not easily doable, so we need some adjustments in Lighthouse.