```
Put the implementation in the cpp, not the h file.
```
Partial backport (4/5) of core [[https://github.com/bitcoin/bitcoin/pull/19290 | PR19290:]]:
https://github.com/bitcoin/bitcoin/pull/19290/commits/8f033642a8c6874184e297b97b951b9bd12ffd75
Depends on D8613.