Adds a new wallet_util.py module and moves generic helper functions
there:
- get_key
- get_multisig
- test_address
This is a partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/15108 | PR15108]] : https://github.com/bitcoin/bitcoin/pull/15108/commits/2d5f1ea2e3f9629d313f1d2aff94a15ef8e1128a
Depends on D6186 and D6185