Needed for future ScriptPubKeyMans which may need to create
SigningProviders dynamically and thus a normal pointer is not enough
This commit does not change behavior.
https://github.com/bitcoin/bitcoin/pull/17261/commits/3f373659d732a5b1e5fdc692a45b2b8179f66bec
Depends on D7861
Concludes backport of Core PR17261