[Enhancement]: Add support to only decouple image tag and repository for CRD resources

Hi,

When using the operator I would sometimes want to set the image tag to whatever the CRD specifies as the default tag used, but change the repository the image is pulled from due to internal policies requiring using private registries.

For now I just specify the repo and tag like described in the docs like so:
`image: mycontainerregistry.io/dragonflydb/dragonfly:v1.21.2`

My suggestion:
```yaml
image:
repository: || defaultValue
tag: || defaultValue
```

So if I only changed the `repository` repository value, the end result would be:
```yaml
image:
repository: mycontainerregistry.io/dragonflydb/dragonfly
tag: defaultTag
```