Implement BE for US3 feature

[AliceeLe]

Estimated effort: 5-8 hours implementing, 2-3 hours debugging
Dependencies: #10
Acceptance criteria: Modularized code that is able to compile and tested upon

[AliceeLe]

I changed the field in the database and designed an API that mark whether the post is answered by instructor or not in public/openapi/components/schema/TopicObject.yml and public/openapi/read/index.yml.

Defining Redis fields in YAML allows for flexible configuration without modifying the application’s code. This is useful in environments where configuration might change depending on deployment contexts

Then, I implemented an API in public/openapi/write/posts/pid/answered.yaml and src/categories/answered.js. However, I am having some roadblocks. Given that we haven’t figured out how to test it, we are slow in that aspect. Furthermore, the BE isn’t connected with the FE yet.