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

[Gholie]

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:

image:
  repository: <someRepo> || defaultValue
  tag: <someTag> || defaultValue

So if I only changed the repository repository value, the end result would be:

image:
  repository: mycontainerregistry.io/dragonflydb/dragonfly
  tag: defaultTag