Add Model format field to Kitfile

To make runtime usage simpler, add Kitfile field .model.format. This can
be set to store the model's data format (e.g. safetensors, gguf, onnx,
etc.).

This information can be used by runtimes to determine how to consume the
model (e.g. if the runtime can handle .gguf files)

pkg/artifact/kitfile.go

@@ -53,6 +53,7 @@ type (
 		Path        string      `json:"path,omitempty" yaml:"path,omitempty"`
 		License     string      `json:"license,omitempty" yaml:"license,omitempty"`
 		Framework   string      `json:"framework,omitempty" yaml:"framework,omitempty"`
+		Format      string      `json:"format,omitempty" yaml:"format,omitempty"`
 		Version     string      `json:"version,omitempty" yaml:"version,omitempty"`
 		Description string      `json:"description,omitempty" yaml:"description,omitempty"`
 		Parts       []ModelPart `json:"parts,omitempty" yaml:"parts,omitempty"`