Add reading and writing example

[dvdsk]

I ran into issues trying to get a duplex pipeline working. It took me a few hours to figure out I should have been using tokio_util::codec::Framed. This PR attempts to add a duplex pipeline example showing:

• How to set up a duplex pipeline
• How to use none SymmetricallyFramed transport to serialize/deserialize to and from a Request and Response struct.
• How to send and receive after each other

[dvdsk]

Started a new project using tokio-serde, couldn't figure out how to do bidirectional framed and ran into this (my own) PR....

@carllerche any chance we could merge this?