Implement loading of keys for DescriptorScriptPubKeyMan

Summary:
Backport of Core PR16528 [16/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/953feb3d2724f5398dd48990c4957a19313d2c8c

Depends on D8413

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8414