Historical retrieval without an entity dataframe

**Is your feature request related to a problem? Please describe.**
The current Feast `get_historical_features()` method requires that users provide an entity dataframe as follows

```python
training_df = store.get_historical_features(
entity_df=entity_df,
feature_refs = [
'drivers_activity:trips_today'
'drivers_profile:rating'
],
)
```
However, many users would like the feature store to provide entities to them for training, instead of having to query or provide entities as part of the entity dataframe.

**Describe the solution you'd like**
Allow users to specify an existing feature view from which an entity dataframe will be queried.

```python
training_df = store.get_historical_features(
entity_df="drivers_activity",
feature_refs = [
'drivers_activity:trips_today'
'drivers_profile:rating'
],
)
```

With the addition of time range filtering.

```python
training_df = store.get_historical_features(
entity_df="drivers_activity",
feature_refs = [
'drivers_activity:trips_today'
'drivers_profile:rating'
],
from_date=(today - timedelta(days = 7)),
to_date=datetime.now(),
)
```