[Chronik] Add OP_RETURN opcode and Script::is_opreturn

Summary:
This helps us identify OP_RETURN scripts.

Depends on D13529.

Test Plan: ninja check-crates

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13530