This sets a handy field for fetching valid infrastructure funding addresses.
`coinbasetxn` is currently unused by mining pools as it is not generated by bitcoind.
In practice, the coinbase transaction is set by the pool software instead of being fetched
from the node.
Should the pool software happen to examine `coinbasetxn`, this change does not interfere with
expected fields as defined in the [BIP22 Spec](https://github.com/bitcoin/bips/blob/master/bip-0022.mediawiki#transactions-object-format).
Depends on D7233, D7272