Add is_running() method

[schloerke]

From @nbenn in https://community.rstudio.com/t/determine-whether-code-is-being-run-as-part-of-a-plumber-api/169128

I am looking for a reliable way of determining if code is being run "under" plumber. Something like what I believe shiny::isRunning() provides.

Thanks!

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I'd like for it to take pr as a value and have it default to something like get_current_router() (or something) that possibly returns the currently running server and checks for a flag.

Could also have a method to return currently running plumber routers (if non-blocking) is implemented. And then the length of the routers could be checked for > 0.