Guide custom theme

[teunbrand]

This PR to the RC aims to fix #5601.

Briefly, it applies the same refactoring in #5554 to guide_custom().

Quick demonstration that the theme inheritance is correct: grey fill from guide's theme and blue outline from the plot's theme.

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  guides(custom = guide_custom(
    title = "My circle",
    circleGrob(r = unit(1, "cm")),
    theme = theme(legend.background = element_rect(fill = 'grey90'))
  )) +
  theme(legend.background = element_rect(colour = "blue"))

^{Created on 2023-12-20 with reprex v2.0.2}

[thomasp85]

LGTM