Add support for rsession-which setting to control default version of R in workbench #19
Description
For some PTD deployments such as academy.posit.team, we want to be able to control the default version of R that is used to match the version in internal.posit.academy.
The PTD site configuration needs to have optional support for setting rsession-which as described at https://docs.posit.co/ide/server-pro/admin/r/configuring_default.html and making that setting appear in the rserver.conf when the setting is present.
Migrated from rstudio/ptd#2190