Add Jupyter plugin support to caliban [draft]

[sritchie]

This PR is here to begin figuring out how to do this for jupyter and jupyterlab.

[codecov]

Codecov Report

Merging #19 into master will decrease coverage by 0.15%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
- Coverage   50.60%   50.44%   -0.16%     
==========================================
  Files          28       28              
  Lines        2897     2898       +1     
==========================================
- Hits         1466     1462       -4     
- Misses       1431     1436       +5     

Impacted Files	Coverage Δ
caliban/platform/notebook.py	42.85% <0.00%> (-2.15%)	⬇️
caliban/platform/gke/util.py	70.76% <0.00%> (-1.16%)	⬇️
caliban/platform/cloud/types.py	89.47% <0.00%> (-0.66%)	⬇️

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[sritchie]

I've looked more into this, and it won't work to do what I've done in the PR! That's because you need to run jupter lab build after you change the directory.

I think we'll need to explicitly support this by adding a jupyter_plugins list to .calibanconfig, and then do this to get them installed without blowing up the base image:

jupyterlab/jupyterlab#4930

If we want to be REALLY pro, we can do a multistage build for this step. That is probably the right approach.