Feature request: accept data.frame as scale labels

[teunbrand]

This is a very niche and selfish problem as I'm trying some stuff out in {gguidance}.
It somewhat relates to #6076 a bit, but I'd like the following code:

library(ggplot2)

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  scale_x_continuous(
    breaks = 2:7,
    labels = data.frame(foo = LETTERS[1:6], bar = 1:6)
  )

get_guide_data(p, aesthetic = "x")
#> Error in `scale_x_continuous()`:
#> ! `breaks` and `labels` have different lengths.

^{Created on 2024-09-07 with reprex v2.1.1}

To return the following data.frame (note that the .label column is a nested data.frame rather than two separate .label.foo and .label.bar columns):

#>           x .value .label.foo .label.bar y
#> 1 0.1127946      2          A          1 0
#> 2 0.2811448      3          B          2 0
#> 3 0.4494949      4          C          3 0
#> 4 0.6178451      5          D          4 0
#> 5 0.7861953      6          E          5 0
#> 6 0.9545455      7          F          6 0

The justification I have for this is that {vctrs} treats data.frames as vectors where rows are treated as elements. We don't really have a good reason to deviate from this behaviour.