chore(api!): standardize api for Backend and BackendExt

[geofmureithi]

Description

This change contains breaking changes in the internal workings of Backend to improve the extensibility and standardize an empty implementation of BackendExt.

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

This does not break the external API and this can be demonstrated by there being no changes in examples