Add miner fund address list to getblocktemplate output

Summary:
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.

Depends on D7233, D7272

Test Plan:

test_runner.py abc_mining_basic abc-magnetic-anomaly-mining mining_basic
test_runner.py --with-axionactivation abc_mining_basic abc-magnetic-anomaly-mining mining_basic

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7235