Adding support to define PodSecurityContext

Integrating the PVC of ActiveMQ Artemis with some storage providers sometimes requires to be able to define the PodSecurityContext which should be used for the Pod.

It should be possible to define an optional SecurityContext in the ActiveMQArtemis deploymentPlan spec, for example:

apiVersion: broker.amq.io/v2alpha4
kind: ActiveMQArtemis
metadata:
  name: example
spec:
  deploymentPlan:
    size: 1
    image: quay.io/artemiscloud/activemq-artemis-broker-kubernetes:0.2.0
    podSecurityContext:
      runAsUser: 1000
      runAsGroup: 3000
      fsGroup: 2000

The Operator should configure the StatefulSet to use the PodSecurityContext in the PodTemplate.

Using mutating webhooks to modify the created Pods. But it would be much nicer to have this possibility built-in.