Feature/update examples (#5)

* add user model and repo;

* rename AuthenticationService -> AbstractAuthenticationService;

* remove unused codes;

* remove unused codes;

* add new migration;

* update models for post and comment;

* update domain logic for user, post, comment;

* add context request to each API to get user_id from token;

* sync event from external authentication service through webhook;

* sync event from external authentication service through webhook;

* fix bugs create post;

* add Keycloak to docker-compose.yaml and docker-compose.dev.yaml;

* add invalid_webhook_secret_error;

* add new config WEBHOOK_SECRET;

* implement check_webhook_authentication;

* handle error for invalid_webhook_secret_error;

* add external_authentication_service_webhook_crud folder;

* Update README.md;

* add examples folder;

* move external_authentication_service_webhook_crud into examples folder;

Author: EdwardPham1615