Implement writing descriptorkeys, descriptorckeys, and descriptors to wallet file

Summary: Backport of Core PR16528 [19/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/46dfb99768e7d03a3cf552812d5b41ceaebc06be

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8417