extend ggplot_image() to other output formats

Prework

• Read and abide by gt's code of conduct and contributing guidelines.
• Search for duplicates among the existing issues (both open and closed).

Thanks for all the work you've done on this package :)

Proposal

Currently ggplot_image() only works for HTML since it uses local_image(). If it were updated to use fmt_image() functionality then ggplots within gt outputs could go to other formats more easily.

library(ggplot2)
library(gt)
library(dplyr)

plot_object <- ggplot(data = mtcars, mapping = aes(x = mpg, y = cyl)) +
  geom_point() 


filename <- "temp_ggplot.png"

# Save PNG file to disk
ggplot2::ggsave(
  filename = filename,
  plot = plot_object,
  device = "png",
  dpi = 100,
  width = 5,
  height = 5
)


gt <- tibble(filename) |>
  gt() |>
  fmt_image(columns = everything(), sep = ",", width = "6in")