Add support for unofficial models

[DannyBen]

Unofficial models require a slightly different payload:

• Instead of model, they expect version with exact authod/model:version_id structure
• The model no longer appears in the API url.

This PR makes the necessary changes to support it:

1. repli generate will now understand YAML files with version and consider them unofficial while YAMLs with model are considered official.
2. repli template new will now understand if the fetched model is unofficial, and create the appropriate template.