Support for initScript in K8s deployment

Hello,

Following a discussion in [slack](https://apache-pinot.slack.com/archives/CDRCA57FC/p1627915642040900) I'm creating this ticket to request support for `init script` capabilities in Pinot's helm chart.

Use-case: Allow defining Pinot resources like `tables` and `schemas` programatically as part of a helm's `values.yaml` file in the form of a reference to a `ConfigMap` resource so that there is a controllable form of defining an initial state for any Pinot deployment.

For inspiration consider Cassandra's approach: https://artifacthub.io/packages/helm/bitnami/cassandra#initialize-the-database

Start by reviewing Pinot's Helm chart and its values.yaml, then compare Cassandra's initialization approach linked in the issue. Clarify how a ConfigMap reference should define initial tables and schemas and what init-script behavior the chart must support. Done means the agreed configuration can reproducibly establish an initial Pinot deployment state.

Written by the indexing model from the issue text.