Fixed shared client balance subscription handling

[nils2525]

Updated the IBalanceSocketClient.SubscribeToBalanceUpdatesAsync() implementation to subscribe using the mode "2". This will result in socket messages for every "Total", "Available" balance update. Each update will then include both numbers (Total, Available)

Excerpt from the docs:

4、Specify "mode" as 2:
accounts.update#2
Whenever account balance or available balance changed, it will be updated together.

(https://www.htx.com/en-us/opend/newApiPages/?id=7ec52e28-7773-11ed-9966-0242ac110003)