have variant of extraInitContainers that uses tpl

**Is your feature request related to a problem? Please describe.**
I want to be able to pass in a string to get evaluated as a template for init containers instead of having to pass everything in as literals.

**Describe the solution you'd like**
It's a common approach to have a variable used as a provided template for specific blocks

**Describe alternatives you've considered**
Providing automation to helm that drives the init container configuration. Maybe cdk8s.

Locate the chart's existing extraInitContainers configuration and inspect how Helm template evaluation is used elsewhere in the chart. Determine how a string-valued variant should be represented and validated, then confirm that templated init-container configuration renders correctly during chart testing.

Written by the indexing model from the issue text.