FR: Add Diagnostic Entities on Scanners

From message from Lash-L — Today at 2:13 PM
> Thoughts on adding a device per each scanner and then adding some entities to it?
> I see on a lot of issues you do a lot of debugging diagnostics. What if some of those attributes became easier for users to see and parse.
> i.e. "Average update interval"

I'm dead keen on this, but haven't worked out how/what exactly yet, so let's try now.... DRAFT FOR DISCUSSION

These would all probably be on a 1 minute or longer update cycle.

- [ ] Proxy Avg Update Interval (or peak avg update interval?)
- answers: What's the best this proxy can do at catching every ad from a device?
- interpretation: 0-2 Excellent, 1-2 Great, 3 Moderate
- search all (recent) hist_intervals for the given proxy, and find the one with the lowest mean().
- report the mean/min/max of that interval set.
- this works by identifying the device that makes this scanner look the best it can, avoiding benchmarking against devices with long intervals or lossy signal paths.
- this is "aliased" by Bermuda's ~1-second update rate, so a proxy can't do better than that.

- [ ] Proxy Reporting Stats
- How well does the proxy forward ads to HA? Do we always have fresh data from the proxy every second?
- Replace the `stale_updates` count, to instead feed a `hist_interval_updates[ fresh, stale]` list. We increment fresh or stale on each update cycle, depending on whether the proxy has given us *any* new data. If update is fresh and stale count is not zero, we first insert a new tuple in the list. Then our list contains pairs of contiguous fresh/stale update counts.
- Entities:
- [ ] Proxy Update Loss (%) = 1 - (sum(fresh) / sum(fresh+stale))
- how often this proxy fails to provide fresh data. Esphome should be 0%. Shelly should be 33% or 0% depending on whether it's rate-limiting is synchronous across all devices or not.
- [ ] Proxy Avg Outage Duration (s) = mean(stale)
- [ ] Proxy Avg Outage Frequency (Hz) = mean(fresh) / sum(fresh+stale)
- multiply by seconds in a day for outages/day

I think we could do something very similar to the proxy stats for devices. It's a little trickier in that "outages" are legitimate for devices, because sometimes they leave home, while proxies aren't expected to. But by trimming the lists based on keeping `sum(fresh)+sum(stale)` below a certain time limit, we get a good "recent stats as of now" measurement, and HA's history of that entity shows it's variation over time - so you'd see your phone performing well, but then doing "poorly" for a few hours because you were out at work, etc.

So for devices, we check if *any* proxy has a fresh update for us and update fresh/stale accordingly.

Something to keep in mind is that proxy entries in the devices{} dict will also be metadevices in future.