Data Product - Externalize the database connection URL as an environment variable

The internal variable DATABASE_URL must be externalized as an environment variable. This allows for flexibility in naming and helps avoid conflicts.

https://gitlab.eclipse.org/eclipse-research-labs/datamite-project/data-sharing/data-product-composition/-/blob/main/src/route_and_database.py?ref_type=heads#L96

/cc @jabefa , @vasisiop