[Enhancement][data-prepper] Support configurable image.registry

[janhoy]

Is your feature request related to a problem? Please describe.
Many organizations do not pull images from docker hub, but always thorugh a private registry (cache). Now, we have to modify image.repository to something like my.registry/opensearchproject/data-prepper.

Describe the solution you'd like
Follow the pattern from the opensearch char and offer a dockerRegistryvalue:

global:
  # Set if you want to change the default docker registry, e.g. a private one.
  dockerRegistry: ""

Describe alternatives you've considered
None

Additional context
None

[prudhvigodithi]

[Triage]
Thanks @janhoy I agree with we should support custom dockerRegistry, can you please contribute to the fix?
@peterzhuamazon @TheAlgo