Refactor the MNIST classifier example using the v1 Python client

[keithmanville]

Refactor the MNIST classifier example with the v1 client

This includes pulling together the required plugins, refactoring the entrypoint yaml, and updating the notebook to use the new client.

The requirements.txt in the examples directory should be updated to include the required dependencies.

Blocked by:

• feat: V1 client update #530

Definition of done

• The MNIST classifier example is refactored and the notebook is updated
• The example jobs run successfully
• The example is merged into dev