Retrieve the optional file associated with a Request

The sever is now able to get any optional file descriptor that might
have been sent over with the bytes related to a request. This patch
leverages this ability by extending the Request structure with an
optional File, which might have been received from the byte stream.

With the Request containing an optional File, we give the consumers of
this crate the possibility to receive a file descriptor associated with
the HTTP request.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

coverage_config.json

@@ -1 +1 @@
-{"coverage_score": 93.1, "exclude_path": "", "crate_features": ""}
+{"coverage_score": 92.8, "exclude_path": "", "crate_features": ""}