[Feature]: support changing plugin-barman-cloud-sidecar image for air-gap envionment

### Is there an existing issue already for this feature request/idea?

- [x] I have searched for an existing issue, and could not find anything. I believe this is a new feature request to be evaluated.

### What problem is this feature going to solve? Why should it be added?

My environment to deploy cnpg is in air-gap so cannot use ghcr.io container registry with below error messages
```
Events:
Type Reason Age From Message
Warning Failed 43m (x257 over 23h) kubelet Failed to pull image "ghcr.io/cloudnative-pg/plugin-barman-cloud-sidecar:v0.5.0": initializing source docker://ghcr.io/cloudnative-pg/plugin-barman-cloud-sidecar:v0.5.0: pinging container registry ghcr.io: Get "https://ghcr.io/v2/": dial tcp 20.200.245.241:443: connect: connection timed out
Normal BackOff 3m42s (x5865 over 23h) kubelet Back-off pulling image "ghcr.io/cloudnative-pg/plugin-barman-cloud-sidecar:v0.5.0"
```

But there's no way to override sidecar image url to use private image registry.

### Describe the solution you'd like

It needs to override sidecar image in cluster crd.

### Describe alternatives you've considered

Cluster CRD like this
```spec:
plugins:
- name: barman-cloud.cloudnative-pg.io
isWALArchiver: true
parameters:
barmanObjectName: my-backup
image: my-private-registry/my-repo/my-custom-barman-cloud-sidecar:TAG
```

### Additional context

_No response_

### Backport?

Yes

### Are you willing to actively contribute to this feature?

No

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct