Add get_online_features method in OnlineStore interface

[tokoko]

I'm proposing we refactor online feature retrieval to move most of the logic for get_online_features and get_online_features_async methods from FeatureStore class in feature_store.py to new methods in OnlineStore class that will have default implementations. This shouldn't be breaking change, nor impact performance in any way, but offers a number of advantages.

• This more closely follows the structure of OfflineStore class that has get_historical_features as one of it's methods.
• Simplifies the implementation of remote online store as all request info will be exposed to OnlineStore provider. Feature Services are really tricky to implement now.
• Related to the point above, simplifies security checks in remote servers as more of request context is available to be passed to the server.
• Will be a step in a direction to address long-standing issue with our online stores that require us making a single round-trip to the database for each feature view because only a single feature view at a time is exposed to online store provider.

[tokoko]

@dmartinol @lokeshrangineni