Allow to place all INPUT_cache folder into a single one when using cache for multi document project #2603
Description
This came up from: quarto-dev/quarto-cli#13494 (reply in thread)
It would be interesting for Quarto project using cache, that may have multiple documents, to be able to centralize all INPUT_cache folder created by rmarkdown configuration of knitr in a single root directory folder different than the default root of the project.
Possibly something like cache.root-dir option to come before cache.path in file.path(<cache.root-dir>/<cache.path>)
As this is again some path resolution and manipulation, this is need to be checked that it would work.
Looking closer to current cache logic and cache.path is important, as maybe this need to be relative to the document always (and not root.dir or else).