Add initContainers option in helm chart

[aturpin1789]

Link to #1134 discussion

I would like to propose adding the possibilities to add initContainers configuration to helm chart.
My goal is to create a container that will pull my rules directory from my gitlab repo and then apply the transformation configured in helm release config file.
I could add another container to do the job but, firstly I can not be sure that rules will be loaded before the transformation are applied and secondly, I do not want a one-shot container created.