Support Dynamic ModelType

[johnmai-dev]

The Swift Package is inconvenient to modify, so I want to change ModelType to dynamic configuration.

Example:

ModelType.registerModelType("Llama5") { url in
    let configuration = try JSONDecoder().decode(Llama5Configuration.self, from: Data(contentsOf: url))
                return Llama5Model(configuration)
}

[davidkoski]

Looks like a nice improvement!