feat: add hetznerdns_nameserver data source

[jkroepke]

Ref: #36 (comment)

[kimdre]

@jkroepke I added a new data source to get the available name servers. Unfortunately there is no api endpoint to retrieve them, only a page in their docs.
What do you think? Should we keep the values all hard-coded? Otherwise we could only keep the names hard-coded and resolve the ip addresses at runtime.

[kimdre]

I changed it so that only the nameserver domains are hard-coded and the the ip addresses get resolved at runtime.