Added the ability to specify additional columns in the `ios_sdk_config.yml` file.

Currently, you could only exclude columns from the Rails backend that you didn't want to persist on the mobile app. This change allows for the opposite, which is to define additional properties that you would want to persist on the app, but not in the backend. For example:

```
User:
  additions:
    - name: active_account
      type: string
```