Refactor custom network classes

# Refactor

## Component(s) to be refactored

- infection_monkey.network_scanning.scan_target_generator

## Explanation
Some custom network classes/tuples are defined for handling IP addresses and IP networks (CIDR). Python already has built-in classes with lots of bells and whistles to handle IPs (https://docs.python.org/3/library/ipaddress.html). Where possible, refactor the Agent's code to use Python's `ipaddress` library instead of custom classes. Remove any disused code.