Add isolatedSymbol parameter to GetMaxBorrowableService

[beansgum]

The isolatedSymbol parameter is required to get the max borrow amount of an isolated margin account.
See https://binance-docs.github.io/apidocs/spot/en/#query-max-borrow-user_data

[codecov-commenter]

Codecov Report

Merging #231 (3a3cc4f) into master (8bc583a) will increase coverage by 2.49%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##           master     #231      +/-   ##
==========================================
+ Coverage   74.78%   77.28%   +2.49%     
==========================================
  Files          55       59       +4     
  Lines        5077     7443    +2366     
==========================================
+ Hits         3797     5752    +1955     
- Misses        927     1245     +318     
- Partials      353      446      +93     

Impacted Files	Coverage Δ
v2/margin_service.go	70.88% <14.28%> (+1.12%)	⬆️
v2/futures/order_service.go	64.30% <0.00%> (-6.18%)	⬇️
v2/futures/exchange_info_service.go	77.11% <0.00%> (-1.71%)	⬇️
v2/futures/websocket.go	4.83% <0.00%> (-1.69%)	⬇️
v2/delivery/websocket.go	4.83% <0.00%> (-1.69%)	⬇️
v2/websocket.go	4.83% <0.00%> (-1.55%)	⬇️
v2/delivery/exchange_info_service.go	85.41% <0.00%> (-1.54%)	⬇️
v2/trade_service.go	86.70% <0.00%> (-0.08%)	⬇️
v2/user_universal_transfer.go	88.23% <0.00%> (ø)
v2/subaccount_service.go	0.00% <0.00%> (ø)
... and 44 more

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[JeremyPansier]

good job