Roborock: Add WiFi signal strength sensor

[SLaks]

Re: https://github.com/rytilahti/python-miio/pull/1914#discussion_r1531240944

All I have is Q Revo and S7 (both cloud connected); I don't know about other devices.

Options include:

• Store prior info() failures and skip this if it fails
• Only call add_update_method(connectivity_status) if the WiFi strength sensor is enabled in HA (requires a more complex refactor)
• Move this sensor to a separate feature in the base class, independent of status classes and everything else (would decouple async work, and may make it easier to implement the previous bullet)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.42%. Comparing base (8643a57) to head (fd973a0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1918 +/- ## ========================================== + Coverage 81.41% 81.42% +0.01% ========================================== Files 193 193 Lines 18636 18648 +12 Branches 4045 4049 +4 ========================================== + Hits 15173 15185 +12 Misses 3180 3180 Partials 283 283 ```

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[SLaks]

Update: As I ran the vacuum, this sensor never changed.

I'm not sure whether that means my implementation is incorrectly caching, or whether the vacuum reports stale data.

[SLaks]

I tried using https://github.com/marcelrv/XiaomiRobotVacuumProtocol/blob/master/network_info.md instead, and watched it print the same value in the terminal as the vacuum moved halfway across the house.

Either my WiFi coverage is literally perfect (which seems very unlikely) or the vacuum doesn't actually make this possible.

Confusingly, my S7 reports 0, even after rebooting somewhere else.