Context

Describe the background behind the problem.

Please see the document, "Design: FEAST Online and Offline Feature Stores" and the EPIC INTPYTHON-297 for details. 

Definition of done

What must be done to consider the task complete?

• Implement all interface methods of OnlineStore
• Test methods by plugging into their "universal" framework via a community maintained mongodb docker container in `testcontainers` package
• Add additional unit test that prove consistency of corner cases
• Add documentation
• Open Pull-Request
• Engage maintainers until this PR is merged.

Pitfalls

What should the implementer watch out for? What are the risks?

None