feat: load configuration from file

* import json config file & update the config (restore)
* export the config and save it to json file (backup)

src/www/js/api.js

@@ -138,4 +138,19 @@ class API {
     });
   }
 
+  async uploadConfiguration(file) {
+    return this.call({
+      method: 'put',
+      path: `/wireguard/upload`,
+      body: { file }
+    });
+  }
+
+  async downloadConfiguration() {
+    return this.call({
+      method: 'get',
+      path: `/wireguard/dl`,
+    });
+  }
+
 }