Add commands for manipulating IPv6 host exposure settings on Arris modems

[phesch]

This PR aims to add a new feature: Four commands for working with the IPv6 host exposure settings available on Arris modems. They are:

• host-exposure:get: Gets the current host exposure settings as JSON
• host-exposure:set <FILE>: Set the host exposure settings on the modem from a file with JSON in the same format as produced by the get command
• host-exposure:(en|dis)able [ENTRY 1 | [ENTRY 2...] ]: Enable or disable all or specific entries in the host exposure settings by name of the entries

I have implemented these commands since they are useful in managing my infrastructure, in the hope that they'll be useful to others as well. Unfortunately, I cannot provide an implementation for Technicolor modems since I don't own one to reverse-engineer the necessary settings with (if those are even available on Technicolor modems in the first place). If someone provides information on how Technicolor modems implement host exposure, I could add a corresponding implementation.