refactor(baselines) Upgrade FedBN Baseline to new flwr format

[maddox-j]

Issue

Description

This PR upgrades the existing FedBN baseline from the previous flwr format to the new version.
Specifically:

1. Removed hydra library dependency
2. Adapted existing configuration files into pyproject.toml format
3. Use of Flower ClientApp and ServerApp
4. Kept manual handling of dataset (discussion points at the end of the PR).
5. Saving of results to disk by overloading the Server class definition.
6. Update README.md with steps to run baseline simulation through flwr run .
7. Result visualisation with Jupyter Notebook.
8. Ensured stateful clients through using the ParametersRecord in the client Context.

Related issues/PRs

#4952 contains a related discussion about leaving the manual handling of the dataset.

Proposal

Explanation

This PR upgrades the existing FedBN baseline from the previous flwr format to the new version.

Checklist

• Implement proposed change
• Write tests
• Update documentation
• Make CI checks pass
• Ping maintainers on Slack (channel #contributions)

Any other comments?

N/A