Use EL ChainConfig in Portal/block proofs

[kdeme]

Use the EL ChainConfig code in any portal or block proofs related code instead of the smaller ChainConfig implementation here: https://github.com/status-im/nimbus-eth1/blob/master/portal/network/network_metadata.nim#L76-L105

The EL ChainConfig code however has some difficulties in usage due to both the posBlock and mergeNetsplitBlock usage. This probably should be revisisted. (I think even that the mergeNetsplitBlock for Sepolia is net set correctly?)