[Chronik] Refactor: Split out parse_* functions into their own module
Summary: These are useful outside of the handlers module, e.g. when subscribing to WebSocket messages on scripts.
Test Plan: ninja check-crates && ninja check-functional
Reviewers: Fabien, #bitcoin_abc
Reviewed By: Fabien, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D13780