[FEATURE] Support uncompressed layers

[sparr]

Description

soci currently does not support layers that are uncompresed.

Describe the solution you'd like

Support these layers

Describe any alternative solutions/features you've considered

No response

Any additional context or information about the feature request

error encountered with an uncompressed layer:

soci: could not build zTOC for layer sha256:[cut]: layer sha256:[cut] (application/vnd.docker.image.rootfs.diff.tar) must be compressed by gzip, but got "unknown": unsupported layer format

command to build an uncompressed layer:

docker buildx build --output type=image,compression=uncompressed

Current workarounds are to rebuild the image with compressed layers, or to exclude the layer from indexing via soci create --min-layer-size

Implementation here should be done in concert with #519 to avoid duplication of effort on adding support for a new "compressor"