examples/: Add file sharing example (#2186)

Basic file sharing application with peers either providing or locating
and getting files by name.

While obviously showcasing how to build a basic file sharing
application, the actual goal of this example is **to show how to
integrate rust-libp2p into a larger application**.

Architectural properties

- Clean clonable async/await interface ([`Client`]) to interact with the
network layer.

- Single task driving the network layer, no locks required.

Cargo.toml

@@ -99,7 +99,9 @@ libp2p-websocket = { version = "0.31.0", path = "transports/websocket", optional
 
 [dev-dependencies]
 async-std = { version = "1.6.2", features = ["attributes"] }
+async-trait = "0.1"
 env_logger = "0.9.0"
+structopt = "0.3.21"
 tokio = { version = "1.0.1", features = ["io-util", "io-std", "macros", "rt", "rt-multi-thread"] }
 
 [workspace]