Introduce multicluster library

[andrewstucki]

This introduces the multicluster library I was spiking on over at https://github.com/andrewstucki/locking. A couple of things to distinguish it from the other library:

1. I attempted to simplify some things by leaving out the multi-kubernetes lock implementation and only bringing over the raft implementation
2. I added a "SingleCluster" implementation that basically just wraps the stock controller runtime Manager. This allows us to immediately wrap our manager initialization code in a multicluster.Manager.

In a follow up I'm going to start introducing some changes to make our reconcilers at least partially multicluster aware, which will be coupled with changes to our factory construction methods so we can initialize connections in other peer kubernetes clusters.