[Enhancement][data-prepper] Support configurable initContainers

[janhoy]

Is your feature request related to a problem? Please describe.

Not possible to configure init containers for the data-prepper pod.

Describe the solution you'd like

User should be able to add their own custom init container for data-prepper to their values.yaml file, through a simple

initContainers:
  - name: foo
    image: busybox
    command: ls

Describe alternatives you've considered

Fork the chart

Additional context

N/A