Dualstack Network Support

[majst01]

Closes #164

Actions Required

The ChildPrefixLength property was removed from the Partition Entity. Every TenantSupernetwork must now be configured with ChildPrefixLength accordingly.
This is done during startup in the database migration step and set to the same value as in the `Partition`.

It is required to add a defaultchildprefixlength property to the tenant super network in you configuration because otherwise the initial network creation will fail:
Sample:
  defaultchildprefixlength:
    IPv4: 22

Also ensure that all firewall-controllers are up-to-date in all firewalls before adding ipv6 prefixes to any of the exiting networks.

Depends on:

• Dual-stack Support docs-archive#202
• Dualstack support metalctl#256
• Dualstack support mini-lab#184
• IPv6 Support metal-core#127
• Support IPv6 metal-networker#110
• Update to metal-networker which allows ipv6 rules on firewall creation metal-images#252
• Dual Stack Support gardener-extension-provider-metal#439
• Validate nftables rule files in CI and fix IPv6 metal-networker#114
• No NAT/MASQ for IPv6 networks metal-networker#119

replaces #544

TODO:

• add possibility to create machines in different networks like internet and tenant-network, but only from tenant-network IPs from both AF are acquired, but only ipv4 is possible from internet even if dualstack

[vknabel]

Fine from my side. If you address the comments from gerrit, this should be ready to go :)

[majst01]

TODO check if required to store AF in the db

[majst01]

Check if this error is really thrown