Handle requests to `/shared` via a custom middleware handler?

[wch]

See https://github.com/rstudio/shiny/pull/3537#issuecomment-1068379288

[cpsievert]

To be clear, we still do mount the entire inst/www/shared directory here (so the title of this issue is a bit of a misnomer):

https://github.com/rstudio/shiny/blob/474f14003b9a1c31d1aefb04d89909d7550c37ee/R/server.R#L388

Although, as https://github.com/rstudio/shiny/pull/3537#issuecomment-1068379288 mentioned, maybe it'd be better to handle requests to /shared/ through a custom middleware handler? It's not clear to me that the benefits would outweigh the complexity cost, though