InfluxDB helm chart - Support initContainers

The current InfluxDB Helm chart provides no way to add additional initContainers and currently the only way to do this is to use Kustomize. In addition to the `extraContainers` field, you should add an `initContainers` field that is the exact same but allows getting placed under the `initContainers:` field.

This is useful for people like me who want to migrate Influx from a docker deployment into kubernetes and initialize the PV's volume before influx starts up by using an init container.

Start by locating the InfluxDB Helm chart's existing extraContainers values and template handling. Mirror that configuration for initContainers, then render the chart with init containers enabled and verify the generated workload places them under initContainers.

Written by the indexing model from the issue text.