Decimal data sent by Avatica cannot be interpreted by some clients

JSON specifies a 'number' data type which does not differentiate between integer and floating-point values. The encoding for JSON numbers is implementation-specific. Libraries such as RapidJSON can treat all non-integer values as 'double' while still being compliant with JSON specifications. As a result, decimal values sent by Phoenix will lose precision on the client side.

Decimal data should be sent as a string. If it is sent as a number, we cannot be expect that all JSON-spec compliant library will handle it properly.

ie. Phoenix sends a decimal as a JSON number, but Rapidjson can only extract JSON numbers as integer or double because the C++ language does not have native decimal support.