[Chronik] Add Display impl for ScriptVariant

Summary: This makes debugging easier, and we can also match the logging easier in D15461.

Test Plan: cargo test -p bitcoinsuite-core --lib

Reviewers: Fabien, #bitcoin_abc, PiRK

Reviewed By: Fabien, #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15462