[backport#16475 1/2] wallet: Enumerate walletdb keys

Summary:
https://github.com/bitcoin/bitcoin/pull/16475/commits/fa6dc7fa5fdfd76c6dd5f7ae693a2fb4e37e271f

Depends on D7019

Partial backport of Core PR16475

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7020