Write `get-file` and commands to get the list of blocks from another node

Description

This closes #1 (closed) by providing an API that allows to get a file with just its hash

Detailed list of changes

• add get_file, get_blocks_info_from and other things related to managing the filesystem
• add new commands to nu API
• remove DragoonBehaviour as it was not useful
• add two new tests for get_file and change the already existing test to use the new API

Note

The get_file is still very rough, the error management needs to be handled better to take into a lot more cases of failure

src/app.rs

-use futures::channel::mpsc::Sender;
-use tokio::sync::Mutex;
+use tokio::sync::{mpsc::UnboundedSender, Mutex};
 
 use crate::commands::DragoonCommand;
 
 pub(crate) struct AppState {
-    pub sender: Mutex<Sender<DragoonCommand>>,
+    pub sender: Mutex<UnboundedSender<DragoonCommand>>,
 }
 
 impl AppState {
-    pub fn new(sender: Sender<DragoonCommand>) -> Self {
+    pub fn new(sender: UnboundedSender<DragoonCommand>) -> Self {
         AppState {
             sender: Mutex::new(sender),
         }