Make compresslevel configurable for .tar.gz

[gdh1995]

What's the problem

The current tar packaging process is not efficient enough, as evidenced by:

• The default gzip compression level "6" can take 200% more time than compression level "1", but the size is only reduced by 10%-20%.
• For tar packages that reach gigabyte levels and are only transmitted and used within an organization’s intranet, packing speed might be more critical than size.

How to solve

Add compresslevel: str which can be "" (auto, 6) | "0" | "1" | ... | "9" (this is extracted from #887).

[cgrindel]

Is there any way that we can test that the compress level value is being passed along to the command line correctly? Alternatively, can we add a test that at least uses the new attribute?

[gdh1995]

Hello, I added a simple test named test_compress_level in tests/tar/pkg_tar_test.py, and its expected sizes are captured on my Python3.8 environment - fortunately it looks like working well in CI jobs.

[cgrindel]

LGTM. I will wait to merge so that @aiuto can review.