Get account balances via rest api

[cryptochassis]

Is your feature request related to a problem? Please describe.
Getting account balances of assets via rest api is important: it allows syncing of information between the exchange and the customer's trading server; it allows algorithms to determine the right action to take knowing the balances.

Describe the solution you'd like
Add an operation type of GET_ACCOUNT_BALANCES to class Request. Add a type of GET_ACCOUNT_BALANCES to class Message. Take bitmex as an example.

• Modify member function void convertReq(http::request<http::string_body>& req, const Request& request, const Request::Operation operation, const TimePoint& now, const std::string& symbolId, const std::map<std::string, std::string>& credential) in include/ccapi_cpp/service/ccapi_execution_management_service_bitmex.h so that its switch statement can handle Request::Operation::GET_ACCOUNT_BALANCES. Use member function appendParam to handle optional parameter of ACCOUNT_ID and ASSET (the name of the asset such as "USD") for operation GET_ACCOUNT_BALANCES.

• Modify member function std::vector<Message> convertTextMessageToMessage(const Request& request, const std::string& textMessage, const TimePoint& timeReceived) in include/ccapi_cpp/service/ccapi_execution_management_service.h so that its switch statement can handle Request::Operation::GET_ACCOUNT_BALANCES and adds a Message to its return. This Message will contain one or more Elements with each Element having a nameValueMap with keys: ACCOUNT_ID (optional), ASSET (optional), QUANTITY_AVAILABLE_FOR_TRADING (required, the amount of the asset available for trading).

• Add appropriate unit tests on request conversions and response conversions.

[cryptochassis]

Reference implementation: https://github.com/crypto-chassis/ccapi/blob/26151970d74dd27177b167834a604071eeb22d2c/include/ccapi_cpp/service/ccapi_execution_management_service_coinbase.h