rpc: Return coinbase flag in scantxoutset
Summary:
Backport of core#26280.
Includes a commit from core#23047:
https://github.com/bitcoin/bitcoin/pull/23047/commits/faca688a8579d7e30d056b6847789fdd56fc0bf4
Depends on D16445.
Test Plan:
ninja check-functional
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D16444