* consensus/beacon: skip extraData validation for OP genesis blocks

The validation added in 0ab188d73 rejects non-empty extraData for
pre-Holocene blocks, but genesis blocks of OP chains have non-empty
extraData. Add ChainConfig.IsOptimismGenesisBlock to identify genesis
blocks (block 105235063 for OP Mainnet, block 0 for all other OP
chains) and skip the validation for them.

* params: add TestIsOptimismGenesisBlock