Make registry client unpacking the default and only `Unpack` implementation

[anik120]

Polling for new content in the remote registry will require an Unpack implementation that it can call and expect the content to be...unpacked.

However, the current default Unpack implementation using pod logs has several issues that looks like potential roadblocks for implementing polling. With a new Unpack implementation being added, polling can be implemented with the expectation from Unpack stated in the first sentence.

All of this is pointing towards ripping out the pod logs based implementation, and making the new direct registry client implementation the default and only one.