Add TypeScript types for middleware

**Feature request**
It would be great to have import for data.

**Use case**
Many users need this functionality for production deployments.

**Implementation idea**
Add a new config option `enable_import` with default `false`.

The title and body describe different changes: TypeScript types for middleware versus a data-import configuration option. Start by locating the middleware type definitions and the configuration handling, then confirm which scope is intended with the issue discussion. Done should be defined by the selected change and include coverage for the stated default behavior if the import option is confirmed.

Written by the indexing model from the issue text.