Implement backups

* trigger backup importation via http route
* follow backup advancement with status route
* import backup via a command line
* let user choose batch size of documents to import (command lines)

closes #884
closes #840

meilisearch-http/tests/assets/backups/v1/metadata.json

@@ -0,0 +1,12 @@
+{
+    "indices": [{
+            "uid": "test",
+            "primaryKey": "id"
+        }, {
+            "uid": "test2",
+            "primaryKey": "test2_id"
+        }
+    ],
+    "dbVersion": "0.13.0",
+    "backupVersion": "1"
+}