The net parameter is a variable representing the network.
The postfix parameter is a string that will be appended to the base
topic before returning the final topic string.
The env parameter is a string that represents the environment in
which the topic is being generated. It is optional and defaults to the value of the CRYPTO_AIO_ENV
environment variable from the process environment.
The function returns a string that is formatted as `crypto-aio.[${env}.]${net}.${postfix}
The function generates a topic string based on the provided network, postfix, and optional environment variables.