Implement the endpoint handler functions inside 'api' package

Currently the controllers under `api` package in the project does not implement any handler functions for request handling. Instead those are simply empty functions. This issue intends to implement the required handler function and creates the mappings between presentation and service layer.

This issue does not concern with the implementation of `/auth` routes or any middleware concerning user authentication, rather this issue tries to fix the basic CRUD functionality using the `/posts` and `/users` route.