Motivation

• Provide a direct approach to start LocalStack for Snowflake by running localstack start snowflake instead of passing the IMAGE_NAME environment variable.
• The same env variable can still be passed to override the image version, though in the future we can introduce a cli option that can be passed to specify versions explicitly, i.e. localstack start snowflake --v 0.3.0. We can even drop the environment variable completely at that point, considering this can be scaled for other platforms as well! Happy to discuss and get more opinions on this!

Changes

• Switched start to a cli group in order to contain other sub-commands while making it available to be run as a standalone command too
• Added snowflake sub-command to support starting LocalStack for Snowflake