add support for mounting in files for opa startup data cli argument

[oc002]

Quick way to add support for mounting in user-defined files into the client pod in order to add support for the files option to work with this helm chart.

[orweis]

@oskar-christensson thanks for the contribution; We'd love to send you some swag as thank you - could you email filip@permit.io your address - so we can send you some? :)