Rework fs filters to be more configurable

[urkle]

Implements the contract like what was suggested in #209 (comment)

Right now implements

• disabling last-modified header exposure
• basic "weak" etag exposure option and etag checking (if match / if none match)
• override read buffer size
• ability to add custom headers in the response
• basic callback to dynamically override options

Outstanding issues

• should I use a boxed Fn<> trait instead of the fn()?
• any other items to expose?
• is the simple weak etag sufficient? or should we do a full etag based on the contents of the files being served?

Sample usage that allows long-cache on js/css assets but forces revalidation via etag of the main index.html (etc) pages which reference those.
This is used to server a VueJS application through warp.

#[tokio::main]
async fn main() {
   // first filter
   // sets no last-modified and enables etags
   // sets a LONG cache-control header on our assets (which cache-busting names)
  let routes = warp::path("assets").and(
            warp::fs::config()
                .last_modified(false)
                .etag(true)
                .add_header(
                    "cache-control",
                    "max-age=31536000, immutable".parse().unwrap(),
                )
                .dir("public/assets"),
        )
        // second filter
        // sets no last-modified and enables etags
        // sets no-cache cache-control header so browser will re-validate the html files in the root :)
        .or(warp::fs::config()
            .last_modified(false)
            .etag(true)
            .add_header("cache-control", "no-cache".parse().unwrap())
            .dir("public"));

  warp::serve(routes)
        .run(([127, 0, 0, 1], 3030))
        .await;
}