Add the JupyterHub extension to the build script

[ravirahman]

• I have read the note above about PRs contributing or fixing extensions
• I have tried reaching out to the extension maintainers about publishing this extension to Open VSX (if not, please create an issue in the extension's repo using this template).

I did not, as this extension is published and maintained by Microsoft.

• This extension has an OSI-approved OSS license (we don't accept proprietary extensions in this repository)

Description

This PR adds the JupyterHub extension to the build script. This extension is OSS under the BSD-3-Clause license

https://github.com/jupyterhub/jupyterhub

https://marketplace.visualstudio.com/items?itemName=ms-toolsai.jupyter-hub

[filiptronicek]

Hi @ravirahman, thanks for your PR. Can you please raise an issue in the upstream repository first? Feel free to use this template: https://github.com/open-vsx/publish-extensions/blob/HEAD/docs/external_contribution_request.md