Allow specifying server prefix and R path

[dometto]

Thanks for this neat package!

I would like to use the setup_rserver function from this module to add multiple RStudio servers to a JupyterHub environment. This can in principle be done by manually calling c.ServerProxy.servers.update({...}) from within a jupyter-server-proxy file. However, currently setup_rserver does not:

1. Allow specifying the server name (prefix)
2. Allow specifying the launcher entry title
3. Allow specifying the path to the R interpreter that should be used

This PR addresses these issues by adding optional keyword arguments to setup_rserver, and by using rserver's --rsession-which-r option when it is available.

This makes it possible to add multiple servers from within a jupyter-server config file like so:

from jupyter_rsession_proxy import setup_rserver
c.ServerProxy.servers.update({
  "rstudio1": setup_rserver(prefix="rstudio1", r_path="/usr/bin/R", launcher_title="RStudio (default R)"),
  "rstudio2": setup_rserver(prefix="rstudio2", r_path="/opt/miniconda3/bin/R", launcher_title="RStudio (other R)")
})

[ryanlovett]

This seems like a useful addition! Ideally jupyter-server-proxy would be able to support multiple instances of an application like /rstudio/0, /rstudio/1, or /rstudio/{name} similar to how jupyter server manages /terminals/, but that would require more work.

One possible issue about the implementation in this PR is the relationship between jupyter-server-proxy's URL prefix as determined by the dictionary key and the prefix parameter that is being added. With this PR they would have to match. If someone (accidentally?) configures:

{"foo": setup_rserver(prefix="bar")},

• RStudio starts with: --www-root-path=/user/username/bar/
• RStudio might generate absolute URLs containing paths like: /user/username/bar/some-page.html
• User accesses: /user/username/foo/ but clicks links to /user/username/bar/

One could try to address this with documentation. Another method is to wrap setup_rserver so that it seeds "foo" as the key and the prefix.

[dometto]

Thanks for the suggestions @ryanlovett! I'm happy to add some documentation, or to wrap setup_rserver, whichever you or other maintainers prefer.

[dometto]

@ryanlovett I've added some documentation now. Let me know if you think this is clear, and suffices.

I've opted against wrapping setup_rserver in another function for now, because I was worried this might lead to a breaking change (given the entrypoint in setup.py would, I think, also have to be changed). But happy to do whatever seems best to the maintaining team!