[Chronik] Use the test framework's CScript in plugins instead of our own version

Summary:
D16544 revealed that to write plugins, a good way to construct Scripts is needed. The test framework already provides such a class, and after moving some functions out of script.py, we have an independent module that we can just import to the plugin system.

Depends on D16550.

Test Plan: cargo test -p chronik-plugin-impl

Reviewers: Fabien, #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16551