Opt class for positional argument handling (#10508)

Opt class for positional argument handling (#10508)

Added support for positional arguments `model` and `prompt`. Added
functionality to download via strings like:

  llama-run llama3
  llama-run ollama://granite-code
  llama-run ollama://granite-code:8b
  llama-run hf://QuantFactory/SmolLM-135M-GGUF/SmolLM-135M.Q2_K.gguf
  llama-run huggingface://bartowski/SmolLM-1.7B-Instruct-v0.2-GGUF/SmolLM-1.7B-Instruct-v0.2-IQ3_M.gguf
  llama-run https://example.com/some-file1.gguf
  llama-run some-file2.gguf
  llama-run file://some-file3.gguf

Signed-off-by: Eric Curtin <redacted>