Add fee recipient to header

[jbearer]

Currently, the fee recipient can be derived from the ChainConfig that applies to a given header. However, this might not always be the case -- maybe in the future the free recipient is the consensus leader, or something, rather than a global config -- and in any case, this makes it annoying for clients like the block explorer to look up. We should just add a new field to the header for the fee recipient, like the Ethereum miner field.