[backport#15427 1/3] Abstract out EvalDescriptorStringOrObject from scantxoutset

Summary:
https://github.com/bitcoin/bitcoin/pull/15427/commits/fb90ec3c33e824f5abb6a68452c683d6ce8b3e4a

Partial backport of Core PR15427

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6616