Allow the OpenAI URL to be configurable using an env var

[oleander]

Add configurability for OpenAI URL using an environment variable and set up a fake OpenAI service in CI workflow.

• Configurable OpenAI URL:

  • Add openai_host field to App struct in src/config.rs.
  • Set default value for openai_host in App::new method.
  • Update src/openai.rs to use openai_host from App struct.
  • Add url subcommand to config openai set in src/main.rs.
  • Implement run_config_openai_host function to handle url subcommand.

• CI Workflow:

  • Add services section in .github/workflows/ci.yml to define a fake OpenAI service.
  • Set OPENAI_API_URL environment variable to http://localhost:8080 in the Run tests step.
  • Configure the service to expose port 8080.