Feature suggestion: labels for VMs and disks (and networks)

### Description

It would be helpful to have labels added to VM instances and disks. Example uses:

- Add roles to servers to use when creating multi-node clusters. E.g., for Kubernetes clusters, we could have control-plane and worker nodes with appropriate labels
- Add creation time and delete-after time to be able to delete VM instances and disks automatically

... and others, similar to what cloud providers do.

It could be implemented as a JSON file in the instance directory or as a BoltDB file, which supports the locking mechanism (using https://github.com/etcd-io/bbolt, of course).

To show the instance (or disk) labels, you would run `limactl list --show-labels.`

Labels could be added to the YAML config file before starting the instance or using `limactl edit.` Also, they could be added using the `--add-labels` flag with `limactl create` or `limactl start.`

Appreciate your feedback.