Add Fetchcode support for mlflow

This is not a standard package type like rpm, deb, or pypi.

There’s no universal way to infer download logic for mlflow-based packages.

repository_url points to an ML API endpoint:

This is a workspace-specific, auth-protected, and non-public URL.
Downloading from it would require:

credentials

Permission to access that workspace

Understanding of MLflow artifact registry API

No download contract:

There is no public or documented API endpoint to reliably get an MLflow model file from a given PURL.

MLflow artifacts are typically stored in internal storage backends (Azure Blob, S3, GCS, etc.), behind auth tokens.