[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