Add ipv6 filter support

[7FM]

This PR adds support to get, create, disable and delete IPv6 filter rules. Besides from that I made some small adjustments that I might move in different PRs (or drop) if desired.

• While analyzing the API for manipulating the IPv6 filter rules, I noticed that for my modem the username 'NULL' is used, so I added a parameter. But it seems to work with 'admin' too, so probably doesn't really matter.
• In the beginning I was unable to login and after longer investigations it turns out that my modem truncates passwords to a length of max. 31 chars. Hence, the login failed because the longer password was sent to the modem which it did not accept. Not sure if this limitation exists for every provider though. So this might be great if others could confirm this behavior.

PS: I have only little python programming experience, so any feedback is welcome^^

[7FM]

seems like my modem received an update tonight and no longer wants that token=<sessionToken> will be send for every post request. I will add an option to disable it, if desired.