Add a method to CWallet to write just CKeyMetadata

Summary:
Partial backport of Core PR14021
https://github.com/bitcoin/bitcoin/pull/14021/commits/bac8c676a727b4216ffcf4c430858bb93fda302e

Depends on D6204

Test Plan:

ninja

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6227