make oras-py behave the same way as oras-go for deciding whether to unpack a layer

[MichaelKopfMkf]

resolves #119

use the same logic as oras-go and cli for determining whether to unpack a tar layer or not.

With this implementation oras-py does not anymore use the layer's media type, it uses the annotation io.deis.oras.content.unpack.

that way artifacts published by oras-cli can be consumed the same way with oras-py and vice versa.

in addition, a skip_unpack param was added to the pull command in favor of #119

[vsoch]

This is great! I think I might have been involved with at least requesting / discussing the feature: oras-project/oras-go#644

I'm waiting for #171 to finish, then likely we will want to merge and rebase off of that. If they work together they can go into the same release (and we can just update the changelog to indicate that).

[vsoch]

okay merged #171 - please rebase here and then we will test.

[stefansli]

hi @vsoch
@MichaelKopfMkf is currently not available. If this is okay for you, I can rebase his changes and create a new PR. Sadly I don't have access to his fork.

[stefansli]

Please see #179. Do you want a rebase or is regular pull+merge okay?