Add Descriptor::IsSolvable() to distinguish addr/raw from others

Summary: This is a partial backport of Core PR14477 : https://github.com/bitcoin/bitcoin/pull/14477/commits/225bf3e3b0a89a285da451cd589be148324039ab

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6125