feat(chart): support additional containers in flagger deployment

[kkkdeon]

This PR adds support for additional containers in the Flagger Deployment.
It allows users to define sidecar containers (e.g., for logging, metrics, or proxies).

additionalContainers:
  - name: sidecar-logger
    image: busybox
    command: ["sh", "-c", "while true; do echo sidecar running...; sleep 10; done"]

[kkkdeon]

Do you have any feedback? @stefanprodan @aryan9600

[kkkdeon]

@stefanprodan @aryan9600
Review would be appreciated.