Provide an annotation function for activities' completing-read

This is what `marginalia` uses to add rich info annotations. I could imagine annotations describing #windows, #files, #buffer, last modified file date, etc. And of course a sort-function which puts activities with most-recently-modified files first.

See the [docs for completing-read](https://www.gnu.org/software/emacs/manual/html_node/elisp/Programmed-Completion.html) for info on `metadata` `category`.