Add on optional field to the `OCIRepository.spec.layerSelector` called `operation` that accepts one of the following values: `extract` or `copy`. When the operation is set to `copy`, instead of extracting the compressed layer, the controller copies the compressed blob as it is to storage, thus keeping the original content unaltered.

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

- Fetch the upstream digest before validation and pulling
- Pull artifact only if the upstream digest is different from the one in storage
- Add the image tag to the revision string `<tag>/<digest-hex>` for a better UX
- Extract the layer processing to a dedicated function

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>