fix: Validate entities when running get_online_features

[franciscojavierarceo]

What this PR does / why we need it:

This PR addresses the issue of unexpected crashes when using feature_store.get_online_features with incorrect or missing join keys.

Specifically, it introduces validation logic in the _get_unique_entities function to ensure that all expected join keys are present and non-empty before proceeding.

If any required join keys are missing or have empty values, a KeyError is raised with a descriptive error message. Additionally, new unit tests have been added to cover various edge cases, ensuring that the function behaves as expected under different scenarios.

Changes:

• sdk/python/feast/utils.py:
  • Added validation for missing and empty join keys in _get_unique_entities.
  • Updated the logic to convert column-oriented table_entity_values into row-wise data and handle edge cases.
• sdk/python/tests/unit/online_store/test_online_retrieval.py:
  • Added a test case to verify that a KeyError is raised when required join keys are missing.
• sdk/python/tests/unit/test_unit_feature_store.py:
  • Imported pytest for testing.
  • Renamed test_get_unique_entities to test_get_unique_entities_success for clarity.
  • Added test cases for missing join keys and handling errors appropriately.

Which issue(s) this PR fixes:

#3270

Misc

It's worth noting, when one key is present and one is not, the retrieval will still succeed as this is expected behavior.

[HaoXuAI]

LGTM!