[backport#15930] rpc: Add getbalances RPC

Summary:
https://github.com/bitcoin/bitcoin/pull/15930/commits/999931cf8f167c7547f1015cdf05437a460c27f0

Depends on D6445

Partial backport of Core PR15930

Test Plan:

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

Check that output is correct

./src/bitcoin-cli getbalances
./src/bitcoin-cli getbalances help

Ditto

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6446