[backport#15730 3/4] rpc: Show scanning details in getwalletinfo

Summary:
https://github.com/bitcoin/bitcoin/pull/15730/commits/d3e8458365ab29017241bc43204fe81cb7fd8530

Depends on D7011

Partial backport of Core PR15730

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7012