[refactor] Use DescriptorImpl internally, permitting access to new methods

Summary:
This is a partial backport of Core PR14646 : https://github.com/bitcoin/bitcoin/pull/14646/commits/24d3a7b3a9e81c04505d130eb7e41885eda9d35f

Depends on D6194

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6195