[Chronik] Plugins: Add Script and Op objects

Summary: This allows users of plugins to parse Scripts with a nice interface that abstracts away all the ugliness generally associated with Script, like handling OP_PUSHDATA1 etc.

Test Plan: cargo test -p chronik-plugins

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D15149