Retrieves a string value from an environment variable and validates it's one of the allowed values.
The name of the environment variable
Array of allowed string values
Optional
Optional default string value to use if the environment variable is not set
The string value from the environment variable if it's in the allowed values list
Will throw an error if the value is not in the allowed values list, or if the environment variable is missing and no default is provided
Retrieves a string value from an environment variable and validates it's one of the allowed values.