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/help_func/exit_func.nu

+export def exit_on_error []: closure -> record<stdout: string, stderr: string> {
+    let res = do $in | complete
+    if $res.exit_code != 0 {
+        error make --unspanned { msg: $res.stderr }
+    }
+
+    $res | reject exit_code
+}
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