[EZ40-49] Add post API

[NielXu]

PR Summary

• GET /api/v1/posts to get all posts
• GET /api/v1/:post_id to get post by id
• DELETE /api/v1/:post_id to delete post by id
• POST /api/v1/posts to create new post

[NielXu]

@XianghuDai @JialeYuLeo @ZhenyeZhu Here is the API for user posts, let me know if additional endpoints needed.