[CHRONIK] add is_final to tx struct in util.rs

Summary: is_final determines whether or not a block is final, and returns a bool dependant on outcome

Test Plan: ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien, tobias_ruck

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien, tobias_ruck

Subscribers: tobias_ruck, Sintayew4, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17155