Add ability to attach additional volumes & volumeMounts to NiFi container

[juldrixx]

From nifikop created by michael81877: Orange-OpenSource/nifikop#147

Feature Request

Is your feature request related to a problem? Please describe.
No.

Describe the solution you'd like to see
Add ability to attach additional volumes to the NiFi cluster image to mount arbitrary configs into the pod :

apiVersion: nifi.orange.com/v1alpha1
kind: NifiCluster
metadata:
  name: simplenifi
spec:
  ...
  volumes:
    - name: secret-params-volume
      secret:
        secretName: secret-params
  volumeMounts:
    - mountPath: /params
      name: secret-params-volume
  ...

Describe alternatives you've considered
The only possible alternative right now is to re-use the existing node-tmp volume via an init container, but you can't pull arbitrary secrets and/or config maps into it. This feature enables that.