Optimize estargz.ReadCloser to not buffer

This is to track [this discussion](https://github.com/google/go-containerregistry/pull/870#discussion_r545241334) (and the underlying issue), which is that to provide the `estargz.Build` interface what it needs we are currently buffer the layer into memory.

We should discuss with the estargz folks whether they *actually* need an `io.SectionReader`, or can be more explicit about the interface they require (which `io.SectionReader` implements). Ideally we could narrow this interface to something that `os.File` *also* implements, but without a standard "length" mechanism that may be hard 🤔