Implement warp::reply::file for dynamic file service

[Molkars]

This pull request implements warp::reply::file, warp::header::conditionals, and publishes warp::header::Conditionals. I also internally moves Conditionals to what I believe is a more fitting place (from fs to header).

Fixes #171
Supersedes #924

[seanmonstar]

Awesome work, thanks! I think I have mainly one suggestion: make Conditionals optional. reply::file(path) could return File, which has an inherent method conditionals() that takes the conditional arguments. What do you think?

[Molkars]

I'm not sure that's very feasible since we return a future dependent on the value initially passed in. We could have a reply::conditional_file though

[mvdschee]

I've been using Molkar's version for a while now, and it works perfectly for my needs. How can we make this PR land into the main version?

[Latawiec]

Any updates here? I'd love to have this feature

[alexander-camuto]

Any updates here? I'd love to have this feature

same

[nickbp]

I'd independently hit this very issue when trying to build a file server that couldn't serve files. I'd imagined that warp would have the building blocks available to allow DIYing this, but AFAICT the only solution right now is to copy/paste a bunch of pub(crate) code out of warp. I got pretty far in doing that, but realized that the exercise had become more hassle than it was worth.

If anyone else stumbles upon this issue after hitting a brick wall like I had, I ended up just switching to using this instead: https://api.rocket.rs/master/rocket/fs/struct.FileServer.html