[FEATURES] Native Sidecar Default Mode + Backward Compatibility

[cheyang]

What feature you'd like to add:
Enable Native Sidecar mode as the default on supported Kubernetes versions to simplify deployment and improve performance. Maintain backward compatibility with legacy Kubernetes by providing automatic fallback to init-container or legacy sidecar injection, with operator override options.

Why is this feature needed:
A default native sidecar strategy enhances deployment simplicity and performance. Maintaining compatibility for older Kubernetes clusters ensures adoption and stability across diverse environments. Automatic selection and explicit operator controls preserve flexibility and reliability during upgrades and version transitions.