RFE: system-setup: new command to install system requirements

Implement new apkg system-setup command to setup host system for packaging. It can be called as part of container setup process.

For now this command will simply install required distro-specific system packages such as devscripts on Debian or rpm-build on Fedora using apkg install.

Provide -I/--isolated option as seen in apkg build --isolated to additionally install requirements for isolated build (pbuilder, dpkg-build-package, ...)

It can perform additional checks/setup as well if needed in the future.

• extend pkgstyles with distro-specific requirements (optionally for --isolated build too)
• implement system-setup command