GetExpectedEnvVarNames is a helper method to list the expected environment variable names that are needed to successfully map onto a given config struct.

NewAppViper constructs a Viper config using the default prefix that we will then extend with additional methods.

NewAppViperWithPrefix constructs a Viper config using a custom prefix that we will then extend with additional methods.

EnvVarPrefix establishes that, by default, our environment variable naming prefix is "APP_".