Add ExtractTarStep and SaveDockerStep

[TadCordle]

Towards #1468 and #1905.

Will probably need to figure out ways to optimize this in a follow-up. For local images, we unfortunately need to compress uncompressed layers and recalculate the compressed layer digests, and this is a bit slow.

For Jib-built images, we name all the layers with their digest, so we could maybe leverage that, but there's no guarantee that this is true for all tarballs (e.g. a docker saved tarball contains uncompressed layers named "layer.tar" in a folder that is named with the uncompressed layer's digest).

Edit: We discussed in the design meeting that it might make sense to save the compressed layer + digest in a folder named with the layer's diff id in Jib's cache; this way we can retrieve the diff ID for a layer in a local base image, use that to check if the layer is in the cache, and only do all the work of compressing/calculating the digest if it isn't.

Also todo: check blob existence on target registry.

[TadCordle]

Note: this method was moved from CacheTest.

[chanseokoh]

Reviewed the first half. I'll look into the rest later.

[chanseokoh]

LGTM, just some minor things.