Implement SignTransaction in DescriptorScriptPubKeyMan
Summary:
Backport of Core PR16528 [28/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/bde7c9fa38775a81d53ac0484fa9c98076a0c7d1
Depends on D8446
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D8447