Retrieves a URL string from an environment variable, validates it, and ensures it ends with a trailing slash.
The name of the environment variable
Optional
Optional default URL string to use if the environment variable is not set
The validated URL string with a guaranteed trailing slash
Will throw an error if the URL is invalid, or if the environment variable is missing and no default is provided
Retrieves a URL string from an environment variable, validates it, and ensures it ends with a trailing slash.