Enrollment: Possibility to switch clients version to download

Client request:
```
Make it possible to override the version tag where the download links point to, per individual OS type
This would be useful when defguard fixes issues and releases and alpha version

For example, now we point our Linux users to the 1.6.0-testbuild2 version, so they don't need to set the timeout to a couple hours, due to the stats/disconnect issue we encountered.
```
@Va1ter I think best would be to add a setting for the Enrollment component and then choose a **channel** for an operating system, eg.:
- Windows: stable/testing
- Linux: stable/testing
- macOS: stable/testing

Start with the Enrollment component and trace how download links and version tags are currently selected for each operating system. Define how per-OS stable/testing channels or version overrides should be configured, then verify that each OS produces the intended download link and add coverage for the selected behavior.

Written by the indexing model from the issue text.