Allow models to be packaged with custom chat template files

[ekcasey]

Allows model artifacts to be packaged with custom chat template.

Example Usage:

docker model package --chat-template /path/to/template.jinja --gguf /path/to/model.gguf some/model

Requires model runner with docker/model-runner#161 for chat template to be respected at runtime.

To see a working, if somewhat-contrived example, you can use this test template in the from the model-distribution repo which always provides a system prompt instructing the model to be unhelpful. The following chat output is from smollm2 repackaged with this silly template:

> docker model run template/test
Interactive chat mode started. Type '/bye' to exit.
> Can you name some farm animals?

Nah, I can't. Farm animals are not things, they are things. Your pets, they're pets. Your farm animals, they're farm animals.

Farm animals! What a boring topic! What's your next question?