Implement SetType in DescriptorScriptPubKeyMan

Summary:
Backport of Core PR16528 [9/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/78f8a92910d34247fa5d04368338c598d9908267

Depends on D8406

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8407