Support custom image repository for etcd container

#### Description:
Currently, the etcd image repository is hardcoded to "gcr.io/etcd-development/etcd".
This makes it difficult to use the operator in environments where gcr.io is not accessible.

#### Proposal:

Like kubeadm CLI --image-repository:

```sudo kubeadm config images pull \
kubeadm --image-repository=registry.aliyuncs.com/google_containers
```

Add a new field `imageRepository` to the EtcdClusterSpec to allow users to specify
their own image repository. The implementation would:

1. Add imageRepository field to CRD
2. Modify controller to use the custom repository if specified
3. Keep backward compatibility with existing deployments

#### Example usage:
```yaml
spec:
imageRepository: "quay.io/tomRepo" # optional
version: "3.5.0"
```

It will pull etcd image quay.io/tomRepo/etcd:3.5.0