Split up key and script metadata for better type safety

Summary:
Refactoring only, no functional change

Backport of core PR11854

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2415