FR: expose mullvad utilization in admin console when assigned by shared pool

What are you trying to do?

When assigning mullvad licenses by group, autogroup, or tag, the admin console does not display utilization.

https://tailscale.com/docs/features/exit-nodes/mullvad-exit-nodes#share-a-pool-of-licenses

"nodeAttrs": [
		{
			"target": ["autogroup:member"],
			"attr":   ["mullvad"],
		},
	],

How should we solve this?

We still track internally which devices have a license, so we should be able to expose a "x of y used" and/or a table of the devices that currently have Mullvad assigned.

What is the impact of not solving this?

Admins are left to reverse-engineer their utilization between referencing the count of devices in each group, autogroup, or tag, against their invoice amount of Mullvad packs.

Anything else?

N/A