Add block reward contract config to ethash and allow off-chain contracts (#9312)

This adds block reward contract config to ethash. A new config `blockRewardContractCode` is also added to both Aura and ethash. When specified, it will execute the code directly and overrides any `blockRewardContractAddress` config. Having this `blockRewardContractCode` config allows chains to deploy hard fork by simply replacing the current config value, without the need from us to support any `multi` block reward scheme.