ggcr: Support streaming already-compressed layers

[bmoylan]

I have a use case where I have an io.ReadCloser representing an already-compressed layer I wish to upload to a registry. As far as I can tell, the only mechanism is stream.NewLayer which expects an uncompressed stream (often from a tarball). The layer will need to compute Digest/DiffID/Size during the single read pass like we do in the uncompressed version. While I could uncompress the input stream with gzip.Reader and provide that as the NewLayer input argument, I do not want the compression parameters to change and want the input's checksum preserved.

Specific case of #1011

// More FR than bug, but I can not remove the label

[github-actions]

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