Allow to select DNS provider type rfc2136

[MartinWeindel]

What would you like to be added:
The dns-controller-manager supports a new DNS provider type rfc2136 and it should made available in the dashboard DNS provider dialogs.
See rfc2136 documentation.
See gardener/gardener-extension-shoot-dns-service#313

Why is this needed:
There seems to be some interest in the community to use DNS servers like PowerDNS or KnotDNS for managing DNS records. Could also can be relevant for private cloud installations.

[MartinWeindel]

Currently, Gardener has no extension supporting rfc2136 for DNSRecord custom resources. Therefore it should not be selectable for the primary DNS provider.