implement index price klines service

[evzpav]

Hi @adshao ! Here is index price klines according to: https://binance-docs.github.io/apidocs/futures/en/#index-price-kline-candlestick-data

Thanks!

[codecov[bot]]

Codecov Report

Merging #405 (7cf9f54) into master (382d09f) will decrease coverage by 0.52%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #405      +/-   ##
==========================================
- Coverage   70.34%   69.82%   -0.53%     
==========================================
  Files          71       72       +1     
  Lines        6694     6744      +50     
==========================================
  Hits         4709     4709              
- Misses       1492     1542      +50     
  Partials      493      493              

Impacted Files	Coverage Δ
v2/futures/client.go	75.74% <0.00%> (-0.76%)	:arrow_down:
v2/futures/index_price_kline_service.go	0.00% <0.00%> (ø)

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