Add various internal packages that will help us in our e2e workflows:

- CLI command bootstrapping
- azure command execution
- remote command execution (SSH + SFTP)
- inventory/state management

Add a series of scripts to be used for building image templates on
Azure. This is accomplished through running 3 scripts in succession:

- 00_check_vm_image: acquire the Azure image URN to use as a base image
- 01_prepare_base_vm: provision required dependencies on the base image
- 02_create_vm_template: turn the customized VM into an image template

Detailed usage and requirements are described in each of the scripts.

This requires versions of ppp and sstp-client which are not yet
available in the Ubuntu archive so we're temporarily using a PPA.

The calling workflows are responsible for passing the required inputs as
secrets to the action.

This workflow runs on a scheduled basis and can also be triggered
manually if needed.

By default it'll figure out the currently supported Ubuntu versions and
build templates for them unless suitable templates already exist (in
other words, templates will only be built when a release transitions
from development to stable, and when no template - stable or
developmental - exists for the release at all).

This behavior can be overridden using the manual workflow inputs, which
provide a way to force refreshing the template builds, and configuring
the matrix of codenames to build.

This set of scripts is responsible for building the current working
directory of adsys as a deb package for the given codename, then
provisioning a VM from our custom VM image template, joining the
integration tests realm and installing the previously built adsys
deb(s).

The package build is done via Docker containers and tests are disabled
both to speed up the build process and to weed out false positives since
adsys package tests were not written to be run in a Docker container.

For Ubuntu releases where adsys doesn't build cleanly, such as jammy, we
provide patches to satisfy the build process.

The structure of these GPOs is described in the ADSys QA Plan document.

The GPOs are added in XML format to be converted to POL before copying
them on the AD controller (using the xml_to_pol.py) script.

We don't measure coverage on end-to-end tests, and the code there
doesn't run in production, so it shouldn't affect our coverage status.

And use apt-get build-dep instead of mk-build-deps.

Previously, the approach was to:
- always build an image if no custom image exists
- build stable image if we only have daily custom images
- never build again if a custom stable image already exists

Now, we take into consideration the build number of the Azure
Marketplace VM image, and ensure we refresh our custom images when new
base images are published. This means we have to track the build number
as part of the custom image version to ensure idempotency of the build.

Thus, the version format of our custom images becomes the following:
- X: major version, 0 for development releases, 1 for stable releases
- Y: minor version, replicates build number of the Marketplace VM, e.g.
     202310110
- Z: patch version, incremented for consecutive builds of the same minor
     version, starts at 0 (in practice this won't be encountered unless
     we force a build with the --force param)

Additionally, the behavior of the 00_check_vm_image script is to no
longer error when there is no image to be printed - this is to be able
to properly differentiate between a valid behavior and an actual error.

Now, the script always exits with 0 unless an actual error occurs. If
there is no image URN to be printed, stdout will be empty so the
consumer will know that the image for the given codename is already up
to date.

And use `keep` instead of `preserve`.

Use eatmydata and the dpkg force-unsafe-io setting to speed up package
installation during base VM preparation.

Pass --unattended to the realm command when joining the domain.

Pass set -eu to the provision script to make it more resilient.

We don't really care how these fields are serialized (as long as they
are serialized), it's also less work when we decide to add/remove a
field.

As opposed to a flat structure.

Prefer apt-get instead of apt in scripts, and only set DEBIAN_FRONTEND
where needed.