Add support for testing on embedded distributions

[tomeon]

Plus a smattering of other features and fixes.

Summary:

1. Add support for running LibreELEC under Vagrant. This includes tooling for building a custom Vagrant box for LibreELEC guests, plus a Vagrant plugin defining the LibreELEC guest type.
2. Add support for running OSMC under Vagrant. This includes tooling for building a custom Vagrant box for OSMC guests, but does not include any Vagrant plugin: OSMC is based on Debian, which already has a suitable Vagrant guest plugin.
3. Add support for Alpine Linux to the Ansible role, add an Alpine Linux Vagrant guest, and add Alpine Linux to the list of distributions tested in the GitHub Actions workflow.
4. Add a contributing guide with information on development tooling.
5. Use the wait_for module (which is for waiting for a particular host) instead of the pause module (which is for pausing Ansible's execution).
6. Use root as kodi_user on LibreELEC,
7. Ensure all groups in kodi_groups exist.
8. Use the service module rather than using the systemd module, and begin migration to using kodi_service rather than kodi_systemd_service. Alpine, among other distribution, does not use systemd for service supervision and management.
9. Add a Nix flake that provides, among other things, the tooling required to build LibreELEC and OSMC Vagrant boxes and to run Vagrant guests based on those boxes.

Thank you for your consideration!

[jose1711]

Thank you!