Live-check v2: emit "not_stable" finding only for stable registries / signals

[lmolkova]

e.g. on the screenshot below I'm running live check against unstable registry. It's more than expected that some attributes/signals are not stable.

But v2 registry manifest will contain stability property, and since we know expected stability of the whole thing, we can warn/error only when stability of an attribute/signal is lower than one of a registry.

[jerbly]

This will be easy to tie in when #1248 is done.