api: standardize Config by adding Backend::Config

[geofmureithi]

Description

api: standardize Config by adding Backend::Config and dropping Backend::Connection

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

Testing

• I have added tests that prove my fix is effective or that my feature works
• I have run the existing tests and they pass
• I have run cargo fmt and cargo clippy

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Additional Notes

• Drops Backend::Connection