Gitlab CI templates for using, building, and publishing conda envs

Add gitlab-ci-templates for automating publishing conda envs to gitlab.

• By including conda.yml, you get a base miniconda env in /opt/conda.
• By including conda-dist.yml, you get two manual jobs added to your pipeline: -- build_conda_env - builds the conda dist env for your project and exposes it as a Gitlab CI Artifact (you can download this from the Gitlab Pipeline UI) -- publish_conda_env - publishes the built conda dist env to a Gitlab Generic Package Registry.

Example usage over in example_job_project: https://gitlab.wikimedia.org/repos/data-engineering/example_job_project/-/blob/master/.gitlab-ci.yml