[RFC] Explicit entity object discovery, lifetimes, and registration

[puddly]

PlatformEntity objects, among others, rely on implicit "registration" inside of __init__ by mutating a global dictionary that ZHA flushes when creating new devices. This is a bit difficult to work with, in my opinion, and is somewhat preventing zigpy/zigpy#1535 from being implemented.

This PR:

1. Construct entity objects early.
2. Implement an on_add interface to mirror on_remove and allow for entity object creation to not rely on any sort of global state.
3. Allow entities to explicitly indicate if they are not supported by the current device via Entity.is_supported (this is mostly a copy/paste of the create_platform_entity logic).
4. Funnel all device entity discovery explicitly via gateway.py.

[puddly]

This PR is "done" architecturally, for the most part.

Main changes:

• All entities must compute capability bitmasks and other feature flags in Entity.recompute_capabilities().
  This is required because we effectively perform multiple passes when setting up entities: an initial pass to discover entities and claim cluster handlers, a second pass to initialize the cluster handlers and possibly populate the attribute cache, and a third pass to actually validate whether or not the discovered entities are applicable to the device. In the future, it would be nice to consolidate this functionality entirely into the Entity subclass.
• Entities indicate device applicability via Entity._is_supported().
  For the same reason as above, since entity objects are created at runtime, we need a way to signal applicability to a device independent of creation. I've also added a new _attr_is_always_supported to allow entities that do not abide by these rules to be easily flagged (quirks v2, group, RSSI/LQI, and device counters).
• All entity objects' listeners, callbacks, and tasks must be registered in Entity.on_add() and deregistered with Entity.on_remove().
  For simplicity, there is also a Entity._on_remove_callbacks list that will be iterated over by the default Entity.on_remove() callback.

That about covers everything. This is a very, very rough first pass PR and is in not in a state for review beyond architecture changes.

[codecov]

Codecov Report

Attention: Patch coverage is 99.35484% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.40%. Comparing base (c23285f) to head (832fc62).
Report is 5 commits behind head on dev.

Files with missing lines	Patch %	Lines
zha/application/gateway.py	90.00%	1 Missing ⚠️
zha/application/platforms/light/__init__.py	97.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #365      +/-   ##
==========================================
- Coverage   96.58%   96.40%   -0.19%     
==========================================
  Files          61       61              
  Lines        9526     9584      +58     
==========================================
+ Hits         9201     9239      +38     
- Misses        325      345      +20     

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