Add NAT API

[SNoof85]

This PR would add NAT API.

[stilllman]

Thanks for the PR! I've merged it after some modifications: I've renamed the functions to be a bit more meaningful and consistent with the other APIs, I've updated the doc slightly to match the one on the Freebox SDK webpage, and more importantly I've added the ability to create/delete port forwarding.

I wanted to push these changes to this PR so we could discuss them, but I didn't have the rights to do so, so I just went ahead and merged it directly 😅

Edit: I just wish there was a way to mark the PR has "merged" instead of "closed" when doing the merge manually, but I can't find a way to do that 🤔

Edit: Seems like I'm not the only one wanting that: isaacs/github#2 😄

[SNoof85]

You're the boss on the project Luc ;)
As you probably know this is mainly for integration to home assistant. The platform is almost ready on my local dev branch. I'll rework it to match your modifications. I did not planned yet adding or removing port forwarding rules from home assistant. That would need to develop a service I guess.
Thanks for your help and your time. And I see it's pushed to pypi ! That's perfect.