[Chronik] Add TxId to bitcoinsuite-core

Summary: Similar to BlockHash, but for TxId. We keep the implementation of both separate so they can't refer to the same type.

Test Plan: ninja check-crates

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13458