Add support for rms_current_ph_b/c

[abmantis]

Add support for 3 phase current attributes from the ElectricalMeasurement cluster.
To avoid adding the 2 extra entities to all devices with an ElectricalMeasurement cluster, this only adds them if the attributes have a value. The downside is that this only adds them after a restart/reload. With #311 this could be improved to match on a 3phase feature instead.

Related PR on HA Core: home-assistant/core#132871

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.48%. Comparing base (c013c5b) to head (2e2e336).

Files with missing lines	Patch %	Lines
zha/application/platforms/sensor/__init__.py	90.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #324      +/-   ##
==========================================
- Coverage   96.49%   96.48%   -0.02%     
==========================================
  Files          61       61              
  Lines        9426     9446      +20     
==========================================
+ Hits         9096     9114      +18     
- Misses        330      332       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[abmantis]

Drafting since test_in_cluster_handler_config is failing