[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.

No files, tests, or entry points are named. Start by locating the answer-evaluation code and the exercise context handling, then inspect how a single correct solution is represented. Done means an exercise can accept any answer from a defined set while preserving the existing behavior for single-answer exercises.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.