T7037: Add prebuild hook for the package-build

[sever-sever]

Change summary

Add prebuild hook for the package-build

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T7037

Related PR(s)

Details

The pre_build_hook is an optional configuration defined in packages.toml.
It executes after the repository is checked out and before the build process begins.
This hook allows you to perform preparatory tasks, such as creating directories,
copying files, or running custom scripts/commands.

Usage:

• Single command:
  pre_build_hook = "echo 'Hello Pre-Build-Hook'"

• Multi-line commands:

    pre_build_hook = """
      mkdir -p ../hello/vyos
      mkdir -p ../vyos
      cp example.txt ../vyos
    """

• Combination of commands and scripts:
  pre_build_hook = "ls -l; ./script.sh"

Example

$ cat package.toml 
[[packages]]
name = "vyos-1x"
commit_id = "current"
scm_url = "https://github.com/vyos/vyos-1x"

pre_build_hook = "echo 'Hello Pre-Build-Cmd'"
build_cmd = "echo 'Hello build-Cmd'"

build

vyos_bld@6bb520f6cf1f:/vyos/work/tmp/T7037/vyos-build/scripts/package-build/vyos-1x$ ./build.py 
Cloning into 'vyos-1x'...
Resolving deltas: 100% (69968/69968), done.
Already on 'current'
Your branch is up to date with 'origin/current'.
I: execute pre_build_hook for the package vyos-1x
Hello Pre-Build-Cmd
I: Patch directory patches/vyos-1x does not exist, skipping patch application
I: Tarball created: vyos-1x_current.tar.gz
dpkg-buildpackage: info: source package vyos-1x-build-deps
...
Hello build-Cmd
I: Cleaned up build dependency packages

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

👍
No issues in PR Title / Commit Title

[zdc]

I would prefer to have a list of commands here - most likely in places where the build customization will be required, in many cases one command will be not enough.

[sever-sever]

I would prefer to have a list of commands here - most likely in places where the build customization will be required, in many cases one command will be not enough.

It works for several commands like this

pre_build_hook = """
mkdir -p ../AA/BB
mkdir -p ../BB
cp example.txt ../BB/
"""