input: Write a python function to sort a list of Employee objects by their salary in ascending order.
  assert sort_employees_by_salary([Employee('Alice', 30, 70000, 'HR'), Employee('Bob', 25, 80000, 'IT'), Employee('Charlie', 28, 75000, 'Finance')]) == ['Alice', 'Charlie', 'Bob']
  assert sort_employees_by_salary([Employee('Dave', 40, 60000, 'HR'), Employee('Eve', 35, 90000, 'IT'), Employee('Frank', 50, 50000, 'Finance')]) == ['Frank', 'Dave', 'Eve']
  assert sort_employees_by_salary([Employee('Grace', 32, 85000, 'HR'), Employee('Hank', 29, 82000, 'IT'), Employee('Ivy', 31, 88000, 'Finance')]) == ['Hank', 'Grace', 'Ivy']
output: def sort_employees_by_salary(employees):
      return [emp.name for emp in sorted(employees, key=lambda e: e.salary)]