feat: add environment variable support for text request body

lib/utils/envvar_utils.dart

@@ -86,6 +86,7 @@ HttpRequestModel substituteHttpRequestModel(
         value: substituteVariables(param.value, envMap, activeEnvironmentId),
       );
     }).toList(),
+    body: substituteVariables(httpRequestModel.body, envMap, activeEnvironmentId),
   );
   return newRequestModel;
 }