Introduce DescriptorCache struct which caches xpubs

Summary:
Backport of Core PR18204 part [1/7] : https://github.com/bitcoin/bitcoin/pull/18204/commits/474ea3b927ddc67e64ae78e08c20c9264817e84d

Depends on D8380

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8381