[FR] Instance Assets: add option to download a file from Non-Public folder

### Description

Hi,
The Assets option in Flowfuse instances is a very welcome addition to the NR\FF bag of tools. As is the option to upload a file to the Assets.

We are using NR with Assets to generate a file for one of our external contacts. The file that is generated is now placed in a non-public folder, however there is no option to download it (without making it publicly available, which is not desired). For testing purposes it would be very convenient to have the ability to download the file directly from the folder (in a FF Admin role).

This option is currently not available, so there is no easy way to get to the resulting file.

Thanks!
DenW

### Which customers would this be available to

None

### Have you provided an initial effort estimate for this issue?

I am not a FlowFuse team member