Allow for custom name and configuration file in setup_rserver

[ticoneva]

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Having the ability to choose between different R versions is very convenient. Because jupyter-server-proxy relies on iterating through entry points in each proxy package, the only way to enable that right now is to modify jupyter-ression-proxy itself.

If setup_rserver allows custom name and configuration file, all it takes to add additional R versions on Jupyter is to create a new skeleton package that:

1. imports jupyter-ression-proxy:setup_rserver and
2. has additional entry points for jupyter_serverproxy_servers.

Here is a working example we currently use on our HPC cluster.

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