v0.5 example models

[alanocallaghan]

The existing example models (eg https://github.com/bioimage-io/spec-bioimage-io/tree/main/example_descriptions/models/unet2d_nuclei_broad) are very useful for testing software that supports previous bioimageio spec versions. However, none of these (to my knowledge) have been updated to use the 0.5 spec, making it comparatively more difficult to check support for both 0.4 and 0.5 models (beyond scouring the model zoo for models that have both 0.4 and 0.5 versions available).