FR: Supporting caption in LabelConfiguration

[IndrajeetPatil]

Currently, we only support the following properties:

LabelConfiguration$new(
  title = NULL,
  subtitle = NULL,
  xlabel = NULL,
  ylabel = NULL
)

It will be nice to be able to also specify a caption and its properties:

library(ggplot2)

ggplot(mtcars, aes(wt, mpg)) + geom_point() +
  labs(caption = "Source: mtcars dataset")

^{Created on 2022-04-29 by the reprex package (v2.0.1.9000)}