Support importing a Deephaven Extension package with hooks in backend and frontend

[mofojed]

See discussion: #1468

Extension needs a directory structure specified, ie:

/MyExtension
  /init
    /grpc-api
    /py-env
  /ui
    /dashboard-plugins

Not too sure what the install command/process would be yet, that is something to discuss here. But in terms of the structure:

• /init folder for initialization scripts. In the matplotlib case, this would need a script in grpc-api to pip install matplotlib and a script in py-env to extend matplotlib.figure.Figure with the getDeephavenObject method. We could add subfolders for any other initialization scripts we may need (eg. for other containers)
• /ui folder for Web UI dashboard plugins. In the matplotlib case, the MatPlotLibPlugin handles it.

[devinrsmith]

I don't think we should try to create a new directory structure for the server side code just for type-extensions - extending the server with any 3rd-party code is a wider issues IMO, and can be handled in a more generic (and just as "easy") way.

At the end of the day, to support 3rd-party code we need to have a way for the 3rd-party to easily add a jar(s) to the classpath, or to install a new python library (which will be lreachable via java/jpy).

[mofojed]

What about installing local/private jars/python libs?

[devinrsmith]

That's all fine - I'm including local/private jars, etc. I'm going to link to some other relevant tickets shortly...