feat: Expose new rate structure property

[stabbylambda]

I have a gnarly template in my home assistant config to reverse-calculate the rate name from the current cost, which is kind of a bummer because Jinja floats absolutely suck.

{% set rates = state_attr('sensor.san_diego_gas_electric_co_current_energy_rate', 'all_rates') | map('round', 3) | list %}
{% set current = states('sensor.san_diego_gas_electric_co_current_energy_rate') | round(3) %}
{% set i = rates.index(current) %}
{% set d = {
  0: "on peak",
  1: "off peak",
  2: "super off peak",
  3: "on peak",
  4: "off peak",
  5: "super off peak",
} %}

{{ d.get(i) }}

While I'm sure this template could be better (I'm no Jinja master), I can't see any way around the gross map('round', 3) call except to expose the index of the selected rate structure. SDG&E's EV-TOU-2 plan has both Summer and Winter periods for On, Off, and Super Off Peak periods.

This PR adds the current_rate_structure from firstof9/python-openei#182 to Home Assistant.

[firstof9]

Can you add to the test_sensors.py to check the sensor output to make sure it's returning what you're expecting?

[stabbylambda]

You want me to write tests?! Nonsense. It works on my machine, after all. 😄

[firstof9]

Please rebase, I fixed the issue pytest found.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.69%. Comparing base (bd795e9) to head (e4a9646).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #81      +/-   ##
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- Coverage   94.96%   94.69%   -0.27%     
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  Files           5        5              
  Lines         397      396       -1     
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- Hits          377      375       -2     
- Misses         20       21       +1     

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