API to query features materialized to offline

Today for features materialized to online(Redis), we provide a simple API to query it.

Some users may want to materialize features to offline store(delta lake) and query it later on. This API doesn't exist yet.

We should add this API so users can query features generated by Feathr.

The API should be consistent with online API:
```
res = client.get_online_features(online_test_table, '265', [
'f_location_avg_fare', 'f_location_max_fare'])

res = client.multi_get_online_features(online_test_table,
['239', '265'],
['f_location_avg_fare', 'f_location_max_fare'])
```

The API can be like(this is still up for discussion):
```
feathrClient.getFeatures([f1, f2], feature_table)

feathrClient.joinFeature([f1, f2], feature_table, observation_table)
```