Implement GetMetadata in DescriptorScriptPubKeyMan

Summary:
Backport of Core PR16528 [31/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/b713baa75a62335ab9c0eed9ef76a95bfec30668

Depends on D8453

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8454