Incorrect type in `EndpointConfig` for `endpoint` and `endpoint_follow` configuration

**Describe the bug**
The `EndpointConfig` class has the following definition:

```
@dataclass
class EndpointConfig:
endpoint: str # partial path to the endpoint e.g. day_category
endpoint_follow: str | None = None # partial path to the follow endpoint
```

which was correct when the project started, however through the lifecycle we have moved to using `OptionalHref` classes to reference the URLs:

```python
class FeatureEventsRef(AppBaseModel):
divisions: OptionalHrefMixin = None
event_groups: OptionalHrefMixin = None
events: OptionalHrefMixin = None
updates: OptionalHrefMixin = None
```

while this won't raise an issue as `href` is a wrapper on `str`, this should be properly retrofitted to represent the right type.

**To Reproduce**
Not applicable as these values are internally populated on discovery.

**Expected behavior**
NA