Add support for gzip-compressed CPIO output

[lavishpal]

This PR implement support for gzip-compressed CPIO archives when the output filename ends with .gz
Detect .gz output filenames and wrap the destination writer with a gzip writer
Ran the relevant go tests

[lavishpal]

cc: @imjasonh

[codysoyland]

Thanks for your contribution!

Is this needed if you can just run the output to gzip?

Can you include tests?

[lavishpal]

implemented based on an existing TODO comment in the codebase "// TODO(mattmoor): Consider wrapping in a gzip writer if the filename ends in .gz"

Happy to add tests and fix the gzw.Close() error handling.

[lavishpal]

also to add some context on the running two separate commands means the full uncompressed output hits disk before being compressed even piping avoids that but still requires gzip to present as a binary which may be not guaranteed on windows or in minimal/distroless build env

[lavishpal]

@codysoyland , i fixed gzw.Close() to return its error explicitly instead of using defer and also added tests covering both the .gz and non-.gz output paths.

[codysoyland]

Thank you for adding tests! I made a few small suggestions.

[lavishpal]

I address all the changes you have suggested. Can you PTAL?