ctr-remote image optimize should support lazy pulling during optimization step

[ktock]

When there are more and more eStargz images used in the community, there will be more cases to re-optimize already-optimized eStargz image.

In this case, we can apply lazy pulling to prepare rootfs for profiling the workload's file accesses. During this profiling, we can download the entire image contents in the background then convert it to the new one after the optimization step.