add support for GitLab Pipelines

[krancour]

Similar to #71 and #72 although I'm not yet sure of what a practical approach is.

[peschmae]

GitLab introduced CI/CD components, as a reusable config, that also accepts some inputs.

While it's mostly a next step from the templates approach that was used previously, it feels more inline with GitHub actions.

I think providing a CI component that can be used, would make this really neat.

There are a lot of examples at https://to-be-continuous.gitlab.io/doc/

I'll try to take a closer look at kargo-render after Kubecon, and might integrate it in our GitLab pipeline. If I do I'll come back to you with the resulting component.