kci-gitlab: Introducing GitLab-CI Pipeline for Kernel Testing

This patch introduces a `.gitlab-ci` file along with a `ci/` folder,
defininga basic test pipeline triggered by code pushes to a GitLab-CI
instance. This initial version includes static checks (checkpatch and
smatch for now) and build tests across various architectures and
configurations. It leverages an integrated cache for efficient build
times and introduces a flexible 'scenarios' mechanism for
subsystem-specific extensions.

[ci: add prerequisites to run check-patch on MRs]
Co-developed-by: Tales Aparecida <tales.aparecida@redhat.com>
Signed-off-by: Tales Aparecida <tales.aparecida@redhat.com>
Signed-off-by: Helen Koike <helen.koike@collabora.com>