Add a new RPC command: restorewallet

Summary:
Change the wallet_backup.py test to use the restorewallet RPC command instead of restoring wallets manually.

This is a backport of core#22541

Depends on D12776

Minor differences from source material are caused by the missing std::filesystem backport

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12777