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

tests/message_exchange_block_per_block.nu

 use ../swarm.nu *
 use ../app.nu
 use std assert
+use help_func/exit_func.nu exit_on_error
 
 # define variables
 let SWARM = swarm create 2
@@ -108,7 +109,7 @@ try {
     swarm kill --no-shell
 
     print "Checking the difference between the original and reconstructed file"
-    let difference = diff $"($output_dir)/../($res_filename)" $test_file
+    let difference = {diff $"($output_dir)/../($res_filename)" $test_file} | exit_on_error | get stdout
     if $difference == "" {
         print "Test successful !"
     } else {