Feat: add virtual cluster

[Somefive]

Migrate https://github.com/kubevela/prism/tree/master/pkg/apis/cluster/v1alpha1 to here. Add "VirtualCluster" resource.

VirtualCluster is generally similar to ClusterGateway, except that

1. It can additionally treat the hub control plane as the local cluster which does not provide extra cluster gateway access.
2. It does not return the credential data for managed clusters while ClusterGateway resource returns that. This is to ensure only metadata can be achieved.
3. The VirtualClusterClient can be used directly instead of calling the Aggregated API to access.
4. Future extensions could be made to the VirtualClusterClient & VirtualCluster, like Create/Update/Delete.

[chivalryq]

LGTM