Add Spa for serving files for single page apps

[davidpdrsn]

I'll write a larger description when I've written some tests.

The idea is to use this with axum like so

Router::new()
    .route("/foo", get(|| async { "GET /foo" }))
    .fallback(
        get_service(SpaService::new("./dist"))
            .handle_error(|_| async { StatusCode::INTERNAL_SERVER_ERROR }),
    );

[davidpdrsn]

Hm actually been thinking that we can make can make something nicer in axum, that doesn't require serving assets at the root which is odd I think.

[jplatte]

I kinda liked this idea, but it wasn't clear to me why it would be a separate service rather than a setting on ServeDir where users configure a fallback in case no file is found.

[davidpdrsn]

I was thinking that a helper giving you this setup in axum might be nicer:

Router::new()
    .nest(
        "/assets",
        get_service(ServeDir::new("dist")).handle_error(internal_error),
    )
    .fallback(any_service(ServeFile::new("dist/index.html")).handle_error(internal_error));