Add current flow support

[rajlaud]

I have a new integration in the works (https://github.com/rajlaud/home-assistant/tree/victron-ble) that tracks a sensor that reports amp-hours. Would be great to add the appropriate units to sensor-state-data. The PR for the integration will add the units to Home Assistant core.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04 🎉

Comparison is base (a90039e) 94.16% compared to head (9baad2b) 94.21%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
+ Coverage   94.16%   94.21%   +0.04%     
==========================================
  Files          12       12              
  Lines         463      467       +4     
  Branches       25       25              
==========================================
+ Hits          436      440       +4     
  Misses         26       26              
  Partials        1        1              

Impacted Files	Coverage Δ
src/sensor_state_data/__init__.py	100.00% <100.00%> (ø)
src/sensor_state_data/library.py	100.00% <100.00%> (ø)
src/sensor_state_data/sensor/device_class.py	100.00% <100.00%> (ø)
src/sensor_state_data/units.py	100.00% <100.00%> (ø)

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[bdraco]

This one fall into the same category as this PR #51 (comment)

[rajlaud]

Sounds good, I'll follow that approach.

As strange as it may seem, amp-hours are a fairly common unit the boat/RV world, so it's possible that down the road another integration will want it. Can always revisit then.

[bdraco]

Sounds good. Happy to revisit later if we have a lot of demand for this 👍