[New feature] Multiple correct answers possible

It could be useful for some exercises to accept any answer from a set of correct solutions.
For example if we look for any root from a function not all of them.
Current fix: Specify in the context what solution to enter.

未指定文件、测试或入口点。首先定位答案评估代码和练习上下文处理，然后检查单个正确解答是如何表示的。完成的标准是：练习可以接受定义集合中的任意答案，同时保留单答案练习的现有行为。

由索引模型根据 Issue 内容生成。