Permit allow-listing environment variables to be passed to R processes

In cloud environments, it's common to use environment variables to pass secrets and other configuration information to otherwise static apps (containers, and similar). ShinyServer currently filters out these envvars. Solutions such as https://twitter.com/ellisvalentiner/status/1175163382401515522 involve modifying the shiny-server.sh script to pipe env variables into Renviron. That's a work around, but it makes the variables static (changes in env variables are not picked up even through app restarts) and requires maintaining custom scripts.

Requesting an allowlist system to allow envvars matching a regex (by name) be passed along to the R session.

Thanks for the consideration!