feat: add next rate structure sensors

[alex-peck]

Implements two new sensors using the changes from this PR firstof9/python-openei#192 in python-openei:

• The next rate structure
• The time the next rate structure will take effect

These values are helpful for planning usage, preconditioning AC, etc.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.50%. Comparing base (bd795e9) to head (9c68abc).
Report is 12 commits behind head on master.

❌ Your project status has failed because the head coverage (88.50%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
- Coverage   94.96%   88.50%   -6.47%     
==========================================
  Files           5        5              
  Lines         397      400       +3     
==========================================
- Hits          377      354      -23     
- Misses         20       46      +26     

Files with missing lines	Coverage Δ
custom_components/openei/const.py	100.00% <ø> (ø)

... and 2 files with indirect coverage changes

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[firstof9]

Looks good, thanks!