Introduce difficuly level for exercises

[ZidongWang123]

Feature: Introduce difficulty level for exercises

  Scenario: Creating exercises
    When an instructor  creates an exercise
    Then the user can specify the difficulty level for the exercise, for different levels, we have different level of instructions：
    - For the "Difficult" level, we only provide basic instructions.
    - For the "Medium" level, we can provide some examples in the instructions.
    - For the "Easy" level, we can provide partial code along with the basic instructions to ask user to complete the code.