Add LoadDescriptorScriptPubKeyMan and SetActiveScriptPubKeyMan to CWallet

Summary: Backport of Core PR16528 [10/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/db7177af8c159abbcc209f2caafcd45d54c181c5

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8408