Interactive prompt for provider

[lionello]

defang compose up    
? Choose a cloud provider:  [Use arrows to move, type to filter, ? for more help]
> defang - The Defang Playground is a free environment for testing only.
  aws - Deploy to AWS using the AWS_* environment variables or the AWS CLI configuration.
  digitalocean - Deploy to DigitalOcean using the DIGITALOCEAN_TOKEN, SPACES_ACCESS_KEY_ID, and SPACES_SECRET_ACCESS_KEY environment variables.

WIP. Currently the prompt is shown even if the actual CLI command doesn't care.