firmware: add tier level to plugins table

[fichtner]

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Improve "tier" visibility by adding it to the plugins table in the GUI as "critical", "supplemental", "community". The trouble is that the information is embedded in the plugin metadata file which cannot be scanned remotely so we could only list tiers from local plugins currently. pkg should have the ability to read metadata, it just needs to be added to the packages themselves and then pkg-rquery should be able to handle it.

[fichtner]

I'm shying away from using the big words for the plugin tiers at the moment, but in general it wouldn't be an issue to change it. Let's try to see how this works in practice first.