Implement GetReservedDestination in DescriptorScriptPubKeyMan

Summary: Backport of Core PR16528 [24/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/f866957979c23cefd41efa9dae9e53b9177818dc

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8427