[backport#17261 13/13] Refactor: Replace SigningProvider pointers with unique_ptrs

Summary:
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

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7863