Add metallb load balancer.

[cryslith]

This adds the metallb load balancer in layer 2 mode. Note that this doesn't provide any sort of high-availability. There's also no changes to spire virt, so to use it with that, the external IP ranges need to be from the same subnet that spire configures on the host interface (i.e. cidr-nodes).

Note: I'm deferring more complete testing of this for a different PR; for now we just verify that the metallb components report metrics at all.

Closes #361.

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[celskeggs]

Very nice work!