[backport#15153] interfaces: Add loadWallet to Node

Summary:
https://github.com/bitcoin/bitcoin/pull/15153/commits/ab288b4e59c52905343a1de628735b5c871d79a1

This is a partial backport of Core PR15153

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6110