Validate singing config in one place

### Description

Validate signing config in a centralized manner and put the values into a class/struct that can't represent an invalid config state, so that we don't need null checks everywhere. Payment attempted to do something similar.

### Additional Information

_No response_

The issue names no files, tests, or entry points. Start by locating the signing configuration validation and the existing payment implementation mentioned in the description; done means validation is centralized and the resulting class or struct cannot represent an invalid configuration state.

Written by the indexing model from the issue text.