I understand that using the "header-includes" option in the YAML front matter of an Rmarkdown document allows the inclusion of LaTeX packages (details here), but how is this done for an Rstudio R presentation?
You can't. If you want to use LaTeX packages, you need to be using LaTeX, and R Presentations don't.
You can generate presentations in PDF that use LaTeX using the Beamer option to R Markdown presentations. With those the methods on https://tex.stackexchange.com/questions/171711/how-to-include-latex-package-in-r-markdown will work.