Add support for inwx dns-01 challenge

# Use case

Request for adding the inwx dns provider as offered by libdns [here](https://github.com/libdns/inwx).

# Your idea for a solution

Adding support should not be so difficult and I'm willing to help implementing it.

- [x] I'm willing to help with the implementation

Start by locating the repository's existing DNS-01 provider integration and reviewing the libdns/inwx provider linked in the issue. Compare the integration points and verify the resulting support can use INWX for DNS-01 challenges; done means INWX is available as a working DNS provider.

Written by the indexing model from the issue text.