make it easier to change the repository for syslog images

[sebhoss]

Is your feature request related to a problem? Please describe.

We want to pull all container images through our internal registry. Currently, we can only do this for the syslog statefulset by specifying the container images ourselves like this:

apiVersion: logging.banzaicloud.io/v1beta1
kind: SyslogNGConfig
metadata:
  name: default
  namespace: logging
spec:
  statefulSet:
    spec:
      template:
        spec:
          imagePullSecrets:
            - name: pull-secret
          containers:
            - name: syslog-ng
              image: example.com/ghcr/axoflow/axosyslog:4.8.0
            - name: config-reloader
              image: example.com/ghcr/kube-logging/syslogng-reload:v1.3.1

I think this is bad because I now have to manage the version of those images instead of letting the operator handle that and I need to verify on each operator update that those container names still remained the same or otherwise risk a misconfiguration.

Describe the solution you'd like

I would love to do something similar to what is possible with the FluentbitAgent resource:

apiVersion: logging.banzaicloud.io/v1beta1
kind: SyslogNGConfig
metadata:
  name: default
  namespace: logging
spec:
  image:
    registry: example.com/ghcr
    imagePullSecrets:
      - name: pull-secret

Note that I have used registry here instead of repository which is used in FluentbitAgent. The idea here is that the final image location is <registry>/<repository>:<tag>

Describe alternatives you've considered

Just configure the STS as in the top example.

Additional context