Allow os_images.x.uri to be a string with gotpl

Bogdan-Adrian Burciurequested to merge
allow-os_images-uri-gotpl into main

What does this MR do and why?

Adjust sylva-units schema to allow for templatizing the URI of images built outside of Sylva, as it just makes sense to allow it.

Related reference(s)

Test coverage

Merge request reports