Add confluent-hub cli init script

[smartjy]

What changes were proposed in this pull request?

I want to install confluentinc/kafka-connect-elasticsearch,
so i added install-plugin.sh and then solved it with this

(Please fill in changes proposed in this fix)

How was this patch tested?

This scripts is constomEnv script like example/create-connectors.sh.
The script will run. before the container is started

  customEnv:
    CUSTOM_SCRIPT_PATH: /etc/scripts/install-plugin.sh
 
  volumeMounts:
  - name: install-plugin
    mountPath: /etc/scripts
 
  volumes:
  - name: install-plugin
    configMap:
      name: install-plugin

If you want to create init-hub-cli, use kubectl like this kubectl create configmap cm-file --from-file install-plugin.sh

(Please explain how this patch was tested. E.g. helm upgrade on minikube, helm install on gke)

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[karthikeyanas]

We support Connector as a first class API in CFK that would internally use hub-client to install the connectors. Why do we need to do this to install a connector? CC: @yangpei1214

https://docs.confluent.io/operator/current/co-manage-connectors.html