[backport#16887 1/3] Abstract out some of the descriptor Span-parsing helpers

Summary:
https://github.com/bitcoin/bitcoin/pull/16887/commits/230d43fdbc41b356700b0d8a6984d69e00279ade

Partial backport of Core PR16887

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6877