Add (back) IPv6 support

[tomdee]

IPv6 support was potentially broken in previous Python versions and support was removed during the Go rewrite. We want to add it back.

I don't think we can until this Docker bug is fixed - moby/moby#28055

When we do, we'll need to make the following changes:
RequestPool - Return the correct pool, poolID and gateway.
For reference the IPv4 reponse is{"PoolID":"CalicoPoolIPv4","Pool":"0.0.0.0/0","Data":{"com.docker.network.gateway":"0.0.0.0/0"}}
RequestAddress - Check whether the request.PoolID is the V6 poolID. If so then take V6 actions.
CreateNetwork - Check the IP6Data for non-Calico IPAM drivers
CreateEndpoint - Ensure that IP address in AddressIPv6 makes it into the Calico endpoint object
Join - Fetch the IPv6 link local address from the host veth and add that as a route into the reponse.