Implement Unlock and Encrypt in DescriptorScriptPubKeyMan

Summary: Backport of Core PR16528 [23/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/a775f7c7fd0b9094fcbeee6ba92206d5bbb19164

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8426