EmployeeContext - 转换OnBoardingRequest为Employee领域对象

[agiledon]

将应用服务接收的消息OnBoardingRequest转换为Employee领域对象，由OnboardingRequestAssembler装配器承担。子任务包括：

• 创建Employee对象
• 生成员工ID(EmployeeContext - 生成员工号 #5 )
• 装配Employee

伪代码为：

    OnboardingRequestAssembler.composeEmployee() {
        OnboardingRequest.toEmployee(employeeId);
        EmployeeIdGenerator.generate() {
            EmployeeRepository.latestEmployee();
            employee.idFrom(sequenceCode);
        }
    }