Store WalletDescriptor in DescriptorScriptPubKeyMan

Summary:
Backport of Core PR16528 [8/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/834de0300cde57ca3f662fb7aa5b1bdaed68bc8f

Depends on D8404 and D8405

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8406