feat(additional containers): add node-level additional containers

[itamar-marom]

Fixes #84

Description

This PR will add the ability to add additional containers at the node level in addition to cluster-level additional containers. It also will validate duplicate container names in the spec.

An example use case is described in the related issue.
Example use:

apiVersion: druid.apache.org/v1alpha1
kind: Druid
metadata:
  name: additional-containers
spec:
  additionalContainer:
    - command:
        - /bin/sh echo hello
      containerName: cluster-level
      image: hello-world
...
  nodes:
    brokers:
      additionalContainer:
        - command:
            - /bin/sh echo hello
          containerName: node-level
          image: hello-world

This PR has:

• been tested on a real K8S cluster to ensure creation of a brand new Druid cluster works.
• been tested for backward compatibility on a real K*S cluster by applying the changes introduced here on an existing Druid cluster. If there are any backward incompatible changes then they have been noted in the PR description.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added documentation for new or modified features or behaviors.

---

Key changed/added files in this PR

• apis/druid/v1alpha1/druid_types.go
• controllers/druid/handler.go
• controllers/druid/additional_containers.go

[AdheipSingh]

LGTM !!

[AdheipSingh]

nit: mainContainer to container

[AdheipSingh]

cc @cyril-corbon