Add getIsolatedMarginAllPairs endpoint

[quserinn]

Add endpoint : https://binance-docs.github.io/apidocs/spot/en/#get-all-isolated-margin-symbol-user_data

[codecov]

Codecov Report

Merging #350 (3bc1fbb) into master (84448eb) will increase coverage by 0.00%.
The diff coverage is 73.33%.

@@           Coverage Diff           @@
##           master     #350   +/-   ##
=======================================
  Coverage   72.49%   72.49%           
=======================================
  Files          63       63           
  Lines        5912     5927   +15     
=======================================
+ Hits         4286     4297   +11     
- Misses       1203     1205    +2     
- Partials      423      425    +2     

Impacted Files	Coverage Δ
v2/margin_service.go	66.96% <69.23%> (+0.06%)	⬆️
v2/client.go	85.11% <100.00%> (+0.11%)	⬆️

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