[backport#15153] wallet: Factor out LoadWallet

Summary:
https://github.com/bitcoin/bitcoin/pull/15153/commits/17abc0fd52510bfeb85a7b5be04001075ec78b6b

Depends on D6084

This is a partial backport of Core PR15153

Test Plan:

cmake .. -GNinja [-DENABLE_BITCOIN_WALLET=OFF]
ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6096