Extend the BaseRepository to support: add

The function is for creating a new entity in the database.

The function signature should like:

```
add(object: T>: T
```

So the subclass can do:

```
TokenRepository.add(token)
```

Possible edge cases:

* Unexpected property: id. (422 HTTP code with error message)
* Maybe there are some cases that database will throw error like, foreign key violation, we should translate this type of exception to : (500, database exception)

---

Some hints:

- Please read our readme for more information/guide/tutorial.
- Here is [an engineering book](https://greenstand.gitbook.io/engineering/) in Greenstand.
- To know more about our organization, visit our [website](https://greenstand.org).
- If you want to join the slack community (some resources need the community member's permission), please leave your email address.