Add COMPRESS_CMD backup option

What does this MR do and why?

Add COMPRESS_CMD backup option and corresponding DECOMPRESS_CMD restore option, to allow sysadmins to override gzip and its default options with anything.

• Resolves #21994 (closed)
• Resolves #322914 (closed)
• Resolves #17197 (closed)
• Related to #329733 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. On MacOS, with GDK already installed
2. brew install pigz
3. Change to your GDK/gitlab directory
4. Backup your local GitLab with COMPRESS_CMD="pigz -c --fast" bin/rake gitlab:backup:create
5. Notice output Using custom COMPRESS_CMD pigz -c --fast
6. Restore with DECOMPRESS_CMD="pigz -cd" bin/rake gitlab:backup:restore

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.