Feature/2 add support for native package builds

[alee-ntap]

What this PR does / why we need it:

Which issue(s) this PR fixes:
Fixes #2

Special notes for your reviewer:
In Gardenlinux, there are two types of native packages: those directly sourced from Debian, such as base-files, and those specific to Gardenlinux, such as gardenlinux-selinux-module.

To accommodate these two types of native packages, two commits are proposed:

• Enhancing the apt_source() function:
  The first commit aims to enhance the existing apt_source() function to support native packages sourced directly from Debian. This enhancement will enable the function to handle the retrieval and installation of native packages from the Debian repository.

• Adding the native_source() function:
  The second commit involves adding the native_source() function to the codebase. This function will be triggered when the pkg.yaml file specifies source: native for a native package. The native_source() function will handle the necessary actions for handling Gardenlinux-specific native packages. Due to the build_source script already handles the source package name, so that we do not need to specific source package name but native is simply enough for our own native package support.

By introducing these commits, the build_source mechanism will be improved. It will no longer require explicitly specifying the source package name for native packages. Instead, using the keyword native will be sufficient to identify and support Gardenlinux's own native packages.

Release note:

[gardener-robot]

@alee-ntap Thank you for your contribution.

[nanory]

LGTM