Plugins support

[aqeelat]

Is your feature request related to a problem? Please describe.
We're currently using streamnative protocol plugins and it would be great if we can just set a list urls or local files in the chart and then have them added in the relevant pods (or all).

Describe the solution you'd like
Possible solutions:

1. adding a top level section in values.yaml called plugins and then we can add the stuff we want to it
2. allowing us to declare custom initContainers (Allow Custom Init Containers #240)

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

1. custom images (was an overkill)
2. using volumeMounts. This a good option but it means we have to download the plugins and prepare the mounted directory ourselves. I'm not sure how this will play with argo but I didn't test it.
3. using converting the chart into a Kustomize and then adding the initContainers. (Yet to be tested) approach 1 or probably approach 2

Additional context
This will be a quality of life improvement for us.

[lhotari]

@aqeelat Contributions are welcome!