Create a proper widget to use with notebooks

[jleibs]

Out jupyter notebooks currently use some awkward hacks that involve returning an html snippet containing a base64-encoded RRD, an iframe pointing at a rerun instance, and some javascript to stitch everything together.

Although this is functional, it's sub-optimal for numerous reasons:

• With hosted notebooks environments, we can run into CORS issues.
• Attaching the RRD to the DOM chokes when files get to big.
• The whole snippet has to be created up front and then sent to the viewer rather that incrementally streaming.

All of the original work predates having our rerun-io/web-viewer npm package, and any of the things we've learned about streaming via the gradio interface.

The right thing to do going forward is to create a proper jupyter widget, which would allow the notebook to host the assets (avoiding CORS issues), and tunneling any data through the proper notebook data-transmission mechanism.

I suspect anywidget might be a good candidate for achieving this. It looks like there are even ways to use anywidget with svelte components: https://gist.github.com/manzt/af46972d7a0a8e870f5228da66c52891

[jleibs]

Proposal for possible steps:

1. Proof-of-concept anywidget implementation: create a standalone package that bundles rerun-io/web-viewer and can be used as a widget that just loads an empty viewer in a notebook.
2. Add APIs to that widget that map to the JS APIs for controlling channels and sending partial RRD blobs.
3. Figure out packaging story -- does this need to be a separate pip package (rerun-notebook)? Are the deps light enough that we can bundle with rerun-sdk or does maturin bundler fight with anywidget bundler?
4. If we bundle with rererun-sdk update rr.notebook_show() to use the new APIs. Otherwise maybe that equivalent code moves to rerun_notebook.show()? Either way, show will still submit all the data as a single blob, but it will do so by having sent the data over a channel as opposed to the current DOM hacks.
5. Design new API that separates out the phases of creating the widget (with optional blueprint) from streaming the data. Ideally this could work within a single cell where the Rerun widget appears immediately when cell execution starts and then the data is sent to the widget over the course of cell execution.