Entity recomputation and add/remove at runtime

[puddly]

This PR introduces two new events to the ZHA device object:

• DeviceEntityAddedEvent(unique_id: str)
• DeviceEntityRemovedEvent(unique_id: str)

And a new method:

• recompute_entities()

recompute_entities() will emit the above events when entities are added/removed at runtime. I've added it to the main device discovery unit test to make sure that it's idempotent.

Future TODOs:

• Re-interview devices after OTA.
• Call recompute_entities on device rejoin to deal with devices that change modes.
• Call recompute_entities after device reconfiguration button is clicked.
• This is too expensive to call on every attribute update but it would be nice to figure out how to do this dynamically, to supersede the above manual calls?

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.98%. Comparing base (0fab9a3) to head (8662227).
⚠️ Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
zha/zigbee/device.py	93.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #517      +/-   ##
==========================================
- Coverage   96.98%   96.98%   -0.01%     
==========================================
  Files          63       63              
  Lines       10559    10607      +48     
==========================================
+ Hits        10241    10287      +46     
- Misses        318      320       +2     

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