Migrate Jupyter Sidepanel extension to pip-based installation

[zhaoyuanTian123]

Currently, the apache-beam-jupyterlab-sidepanel extension is installed via the deprecated jupyter labextension install mechanism. JupyterLab 4+ officially recommends distributing extensions as prebuilt Python packages (pip-installable).

[Eliaaazzz]

.take-issue

[Eliaaazzz]

I would like to take this issue.
Here is my plan for the migration:

Restructure the project: Transition to a modern Python packaging structure using pyproject.toml and hatch-jupyter-builder.

Update package.json: Configure the jupyterlab metadata to specify the outputDir for the prebuilt assets.

Federated Extension: Shift from the deprecated source extension model to the federated (prebuilt) model. This will allow users to install via pip without needing Node.js/npm at runtime.

Cleanup: Remove outdated installation scripts and update the documentation/README to reflect the new pip install workflow.

Validation: Test the installation in a fresh JupyterLab 4.x environment to ensure the sidepanel loads correctly.