Add MLflow "download artifacts" command to CLI for CI/CD model deployment

[amesar]

Event though "download artifacts" is not part of MLflow REST API (because much of the logic is client-side), it is an important method that is part of the Python MLflow API and the MLflow CLI. This is needed by CI/CD pipelines that need to download a model in order to deploy it to another environment.

Python API: https://mlflow.org/docs/latest/python_api/mlflow.artifacts.html#mlflow.artifacts.download_artifacts

MLflow CLI: https://mlflow.org/docs/latest/cli.html?highlight=download%20artifacts#mlflow-artifacts-download

mlflow artifacts --help

  Download an artifact file or directory to a local directory. The output is
  the name of the file or directory on the local filesystem.

  Either ``--artifact-uri`` or ``--run-id`` must be provided.

Options:
  -r, --run-id TEXT         Run ID from which to download
  -a, --artifact-path TEXT  For use with Run ID: if specified, a path relative
                            to the run's root directory to download
  -u, --artifact-uri TEXT   URI pointing to the artifact file or artifacts
                            directory; use as an alternative to specifying
                            --run_id and --artifact-path
  -d, --dst-path TEXT       Path of the local filesystem destination directory
                            to which to download the specified artifacts. If
                            the directory does not exist, it is created. If
                            unspecified the artifacts are downloaded to a new
                            uniquely-named directory on the local filesystem,
                            unless the artifacts already exist on the local
                            filesystem, in which case their local path is
                            returned directly