[backport#16239] wallet/rpc/getbalances: add entry for 'mine.used' balance in results

Summary:
https://github.com/bitcoin/bitcoin/pull/16239/commits/53c3c1ea9e20f881c843a9219e48cec202e962f8

Partial backport of Core PR16239

Test Plan:

ninja
./src/bitcoind -regtest -daemon
./src/bitcoin-cli -regtest getbalances help

check that it looks right

test_runner.py wallet_avoidreuse

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6482