Add Home Assistant entity description for interval

[bachya]

Describe what the PR does:

SSIA

Does this fix a specific issue?

Fixes #567

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md with any new documentation.

[codecov]

Codecov Report

Merging #571 (9166366) into dev (a0e5450) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev      #571   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           32        32           
  Lines         1707      1712    +5     
=========================================
+ Hits          1707      1712    +5     

Impacted Files	Coverage Δ
ecowitt2mqtt/helpers/device.py	100.00% <ø> (ø)
ecowitt2mqtt/helpers/publisher/hass.py	100.00% <ø> (ø)
ecowitt2mqtt/const.py	100.00% <100.00%> (ø)
ecowitt2mqtt/data.py	100.00% <100.00%> (ø)
ecowitt2mqtt/helpers/calculator/time.py	100.00% <100.00%> (ø)