RTE - File Download - allow custom download logic

## Description

There are cases, when we need to extend file download behavior. Examples:

- to check files for viruses, before allowing user to download it, we need to make an additional API call
- to download files from another domain, we also need additional API call, to make pre-signed URL to another domain

## Describe the solution you'd like

Add a new prop somewhere in RTE config, accepting callback like `onDownload: (link: string) => void`. This callback should be called when the user clicks download. Passing this callback should also disable the default behavior.