Support authentication for private CRAN-like repositories

[SamEdwardes]

This request is inspired by rstudio/renv#307. Renv has a nice way to set authentication headers https://rstudio.github.io/renv/articles/renv.html?q=renv.download.headers#authentication-with-custom-headers

options(renv.download.headers = function(url) {
  if (grepl("^https://my/repository", url))
    return(c(Authorization = Sys.getenv("AUTH_HEADER")))
})

Is there a way to do this in packrat?