Add a prune-instances command #991

What needs to get done

• Add a new command, prune-instances that removes all non-base instances for the active provider using the ProviderService. The following parameters are optional:
  • --all-providers to prune instances from all providers
  • --provider=<provider> to prune for a specific provider
    • If this isn’t specified, use the same business logic that lifecycle commands use to determine the provider.
  • --include-templates to prune base instances
• Unit tests
• A testcraft spread test that runs this command to prune LXD and Multipass instances

Why it needs to get done

To allow users to prune instances.

Requires canonical/craft-providers#639
Requires canonical/craft-providers#893
Required for canonical/charmcraft#1042