Featurizer.get_keys() does not honor candidate classes in context

[HiromuHota]

Description of the bug

Unlike other methods (eg Featurizer.drop_keys() and Featurizer.upsert_keys(), Featurizer.get_keys() does not honor candidate classes in context but returns all feature keys no matter which candidate class they are associated with.
This is confusing.

See #511 (comment) for how this actually confused a user.

To Reproduce

This is a design error.

Expected behavior

These methods should behave similarly. Either

• None of these honor candidate classes, or
• All of these honor them.

Error Logs/Screenshots

N/A

Environment (please complete the following information)

• Fonduer Version: 0.8.3

Additional context

Add any other context about the problem here.