Segregate `Base` model into respective entities.

**Description**
Currently we have just one `Base` class which is inherited by all out models. It contains functionalities common to all class, such as, `Id`, `creation`, `modification` etc.
Now, instead of it we would want to decouple the entire `Base` and segregate it into individual models, such as
- `TimeStampedModel`: For `creation` and `modified`.
- `IdModel`: For `Id`.
..etc.

**File to change**: [Base.java](https://github.com/harsh020/conduit-java/blob/master/src/main/java/com/example/conduit/base/Base.java)