[r-rig] Additional repository option (as like `r-packages` having)

[eitsupi]

For the past month, we have had problems running the r-rig Feature because the httpgd package had been removed from CRAN.
It would be worthwhile to set up something like the recently launched https://github.com/r-multiverse as an additional repository so that packages that have been removed from CRAN can still build properly.

[eitsupi]

httpgd has been removed from CRAN again, so this is neeeded I think.