Add Economic Indicators

[viceroypenguin]

This PR adds support for the remaining Economic Indicator APIs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1eba8ad) 76.45% compared to head (b8be428) 76.80%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
+ Coverage   76.45%   76.80%   +0.35%     
==========================================
  Files          39       40       +1     
  Lines         463      470       +7     
  Branches       27       27              
==========================================
+ Hits          354      361       +7     
  Misses        100      100              
  Partials        9        9              

Files	Coverage Δ
.../AlphaVantage.Net/Indicators/AlphaVantageClient.cs	100.00% <100.00%> (ø)
Source/AlphaVantage.Net/Indicators/CpiRequest.cs	100.00% <100.00%> (ø)

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