Export alpha argument in element_rect()

[Yunuuuu]

I would like to request the addition of an alpha argument to the element_rect() function in ggplot2. Adding an alpha argument to element_rect() would make it easier for users to adjust the transparency of rectangular elements without needing to manually calling ggplot2::alpha() or ggplot2::fill_alpha()

[teunbrand]

For some more context: #6249 (comment)

I don't have any strong opinion on whether this should be exposed, but if we do it for element_rect(), we should do it for all elements.