Support federated extensions

Problem

The webpack build(s) already take(s) a long time. As a site publisher adds additional extensions, this will take longer until it just doesn’t work anymore: for example, while published on npm, ipydrawio doesn’t reliably work as a source extension because… 5000 files. Further, and for the reason above, unless they really need it, I don’t think a lot of extension authors will even be testing source installs.

Proposed Solution

Add federated_extensions to pageInfo and the machinery to load them.

Indeed, the jupyterlite kernels, etc. would likely be useful outside of a jupyterlite install, e.g. when someone is building content, so they gain access to things like jupyter-lsp, etc. but that’s probably for another day, and maybe counterfeited by a preview mode on #41, if the complexity is too high (because patching ServiceManager).

Additional context

For pure js builds, provide some mechanism to get sdists from pypi.io by URL (or locally), unpack them, copy the files into lab/labextensions/.

For python-driven builds (#41), be able to copy things from $PREFIX/share/jupyter/labextensions to lab/labextensions (https://github.com/jtpio/jupyterlite/issues/16#issuecomment-821980005).

[jtpio]

Some ideas for having server extensions as federated extensions:

• a cookiecutter-jupyterlite repo similar to https://github.com/jupyterlab/extension-cookiecutter-ts (maybe a fork?), but for JupyterLiteServerPlugin plugins (example: https://github.com/jtpio/jupyterlite/blob/main/packages/javascript-kernel-extension/src/index.ts)
• distributed as a Python package, with the prebuilt assets going under $PREFIX/share/jupyter/liteextensions
• the jupyterlite cli would also be able to install these extensions from $PREFIX/share/jupyter/liteextensions by moving them to lite/extensions
• extension authors use the same workflow as for normal jupyterlab extensions, with @jupyterlab/builder under the hood. Maybe we can even leverage the --core-path option to not have to add another lite layer on top of it.
• extension authors can then publish their server extension to PyPI, and simply pip install it when they follow a Python drivn deployment

[bollwyvl]

I think we’re pretty good here, I’ll open (ha) the OpenAPI thing as a separate issue.