Move Metering cluster formatting out of cluster handler and into sensor entity

[puddly]

When working on #557 I ran into a bug: diagnostics could not be regenerated for a specific device. For some reason, the cluster handler-specific initialization code did not run.

The cluster handler computed something that realistically should be handled by the entity so I ported the code over into there. This has the benefit of now allowing the summation formatting to change at runtime.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.91%. Comparing base (76550e0) to head (9b2d5ed).
⚠️ Report is 10 commits behind head on dev.

Files with missing lines	Patch %	Lines
zha/application/platforms/sensor/__init__.py	93.10%	2 Missing ⚠️
zha/application/platforms/sensor/helpers.py	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #565      +/-   ##
==========================================
- Coverage   96.95%   96.91%   -0.04%     
==========================================
  Files          63       63              
  Lines       10534    10544      +10     
==========================================
+ Hits        10213    10219       +6     
- Misses        321      325       +4     

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[TheJulianJES]

Hmm, interesting that it affected this Tuya valve with a v2 quirk. We have a local Metering cluster for that and it's one of the few times we actually do not use kW(h) as the unit for it, but instead water/liters:
https://github.com/zigpy/zha-device-handlers/blob/928268cca5d22a048e72d45ae1c24a1b6dd92a9c/zhaquirks/tuya/builder/__init__.py#L128-L148

[TheJulianJES]

Looked through the endpoint/ch initialization code and we skip important parts if "skip configuration" is enabled, both binding and attribute reporting in ch.async_configure but also the ch specific init code in ch.async_initialize, which later runs the ch specific init.
And sure enough, the quirk enables it: zhaquirks/tuya/tuya_valve.py#L115

So, that at least solves the why behind the cluster handler specific initialization code not running.

Moving the Metering cluster formatting out of the cluster handler might actually break the entity displaying the delivered summation, now that the code is executed.
But agreed this is something we should do.