[Chronik] Move+rename UtxoOutpoint to DbOutpoint to data.rs

Summary: This struct is useful outside of UTXO handling (e.g. for plugins), so we move it to a more general place.

Test Plan: cargo test -p chronik-db

Reviewers: Fabien, #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16528