Allow to send messages to a specific target

[herr-seppia]

Describe what you want implemented
The Peer API should expose a send method to "broadcast" a message to a specific target.

Describe "Why" this is needed
This is required to handle a ping/pong paradigm useful in the following scenario
Eg:

• The client caller ask to broadcast a FindBlock message
• Receives multiple BlockReady responses
• Replies to only one of the previous responses with a GetBlock message
• The latter will send back a Block message

Describe alternatives you've considered
The client caller can broadcast a sort of "Ping" message to the network and handle the multiple response.

Additional context
Add any other context or screenshots about the feature request here.

[t00ts]

Does this function definition seem adequate? @herr-seppia

    /// Send a message to a peer in the network.
    pub async fn send(&self, peer: PeerInfo, message: &[u8])

[herr-seppia]

Does this function definition seem adequate?

Does the client know anything about PeerInfo?
Due to the fact this is a prerequisite for #59, maybe a SocketAddr is enough

What about this signature?

    /// Send a message to a peer in the network.
    pub async fn send(&self, message: &[u8], target: SocketAddr) {