(tidyverse approach) calculating rowsum across several columns where info on columns to include comes from a different data frame

Question

Assume the following data:

dat <- data.frame(x1 = c(1, 2, 3, 4, 5),
                  x2 = c(2, 3, 4, 5, 6),
                  x3 = c(3, 4, 5, 6, 7),