Impl IntoResponse for File

[zacharygolba]

Refactoring the static-server middleware can come in 2 phases. First, we can implement IntoResponse for File in via proper. In the future we can decide whether or not to implement an official static server middleware. I like the idea of seeing what the community comes up with.

The criteria for closing this issue is as follows. Create an impl IntoResponse for std::fs::File and tokio::fs::File (both requiring a feature flag). The implementation should respond with the entire contents of the File buffered in memory and should error if the File is larger than some constant limit imposed at the callsite with builder pattern similar to actix.

A streaming file implementation can be added either as a part of one builder struct or two discrete types. It can also be done in a separate pull request with a separate issue. I'm open to bikeshedding about the best approach here if anyone wants to chime in.