Support publication of gzip tar archives

[ncoghlan]

Tar archives are currently always compiled as xz for the improved compression ratio and to avoid the reproducibility problems that arise from the creation time timestamp that's automatically included in the gzip headers.

This can be a hassle if the consuming application can't readily handle unpacking xz-compressed tar archives, so the tar compression format should be configurable when publishing layer archives.

To avoid reproducibility problems arising from python/cpython#75707, the fileobj parameter to tarfile.open will need to be used to pass in a gzip file with the clamped mtime set in the header.

[ncoghlan]

CLI --format options from https://conda.github.io/conda-pack/cli.html#cmdoption-conda-pack-format: zip, tar.gz, tar.bz2, tar.xz, tar

• No infer: output archive names aren't specified directly, so the format can't be inferred from the archive name
• No no-archive: local-export is a dedicated command (distinct from publish)
• No tar.zst: no zstd support (could be added, but not yet requested)
• No abbreviated formats (txz etc): might give the impression that the setting will alter the archive file extensions

Also accept default to specify the platform default (zip on Windows, tar.xz elsewhere)