Make send_file example streaming

[seanmonstar]

The send_file example currently reads the entire file into memory. It would be better to stream the file, to show how to do so in case of large files.

• Instead of using tokio::fs::read(), we can use `tokio::fs::File::open()``.
  • Then, the File can be wrapped in a tokio_util::io::ReaderStream.
  • That stream can be converted into a body with http_body_util::StreamBody.
• But the "not found" body is not a stream, it is still a Full<Bytes>. So, to be able to return both, the final body type needs to a http_body_util::combinators::BoxBody<Bytes, std::io::Error>. You can use http_body_util::BodyExt::boxed() to create one.

[acedogblast]

How would we add tokio_util as a new dependency?
I get this error when building the example.

[seanmonstar]

You can add tokio-util to the [dev-dependencies] in the root Cargo.toml.

[seanmonstar]

Done in #3433.