add difftime and/or hms method to rescale()

[ccsarapas]

This request is prompted by an issue in ggplot2::scale_*_time() described here. Essentially, calling scale_*_time() with the sec.axis argument set throws an error because scales::rescale() doesn't have hms or difftime methods:

library(hms)
library(ggplot2)

x_datetime <- as.POSIXct(paste0("2023-01-01 ", 0:23, ":00:00"))
x_time <- as_hms(x_datetime)
y <- sin(pi * 0:23 / 6)
dat <- data.frame(x_datetime, x_time, y)

ggplot(dat, aes(x_time, y)) +
  geom_line() +
  scale_x_time(sec.axis = sec_axis(trans = identity))
#> Error in UseMethod("rescale"): no applicable method for 'rescale' applied to an object of class "c('hms', 'difftime')"

As @teunbrand demonstrated in the ggplot2 issue, defining a rescale.difftime() method would resolve the issue.