Add model download selector

[HendricksJudy]

Fixes #28

Add model download selector and update model loading process.

• sam2-cli/MainCommand.swift

  • Add an option to specify the model directory.
  • Modify the run function to use the specified model directory for loading models.

• SAM2-Demo/Common/SAM2.swift

  • Add a function to download the model snapshot using the snapshot function from HubApi.swift.
  • Modify the loadModels function to call the new download function before loading the models.
  • Add a new load(from directory: URL) function to load models from a specified directory.