Implement introspectable switches

[rytilahti]

Continuing on adding more introspectable properties (follow-up to #1488), this PR adds support for toggleable switches and implements them for PowerStrip.
The home assistant using this information is available at https://github.com/rytilahti/home-assistant/tree/xiaomi_miio/feat/entities_from_upstream

Here is how the powerstrip looks currently in homeassistant:

Next step is adding support for settings (select platform), but it may take a bit longer as I don't have devices to test on prior cleaning up the vacuum platform integration to use the common facilities.

Ping @syssi @Kirmas @starkillerOG - feel free to test and start adding switches to integrations. Let me know if you encounter any issues or think that something should be changed, we can still adjust the API as needed.

[codecov-commenter]

Codecov Report

Merging #1494 (1d6496c) into master (d872d55) will increase coverage by 0.00%.
The diff coverage is 90.32%.

@@            Coverage Diff            @@
##           master    #1494     +/-   ##
=========================================
  Coverage   82.59%   82.59%             
=========================================
  Files         142      142             
  Lines       14009    14033     +24     
  Branches     1583     3345   +1762     
=========================================
+ Hits        11571    11591     +20     
- Misses       2219     2222      +3     
- Partials      219      220      +1     

Impacted Files	Coverage Δ
miio/powerstrip.py	94.30% <62.50%> (-2.31%)	⬇️
miio/descriptors.py	100.00% <100.00%> (ø)
miio/device.py	56.62% <100.00%> (+1.41%)	⬆️
miio/devicestatus.py	96.00% <100.00%> (-4.00%)	⬇️

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