refactor: Split InterpretOption into Interpret{Key,Value} functions

Summary:
Co-authored-by: Anthony Towns <aj@erisian.com.au>

This is a partial backport of core#22766
https://github.com/bitcoin/bitcoin/pull/22766/commits/26a50ab322614bceb5bc62e2c282f83e5987bad8

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18259