Roll out new unified standards to Notification Controller

[pjbgf]

High-level points covered on this:

• Roll-out new Flux Controller best practices, based on K8S API Conventions.
• KStatus aligned with the one implemented on source-controller.
• Improved code testing coverage.

References on new unified standards:

• Controller Patterns
• kstatus conditions
• kstatus - conditions and generations check

Implements fluxcd/flux2#1601