[clarity] add the ability to query PoX reward addresses from `get-burn-block-info?`

[jcnelson]

Make it so get-burn-block-info? pox-addrs BURN_BLOCK_HEIGHT returns a (list 2 { version: (buff 1), hashbytes: (buff 20 }) that encodes the PoX reward addresses that were paid to at BURN_BLOCK_HEIGHT in this fork.