driver: `ListSnapshotsResponse` should return []struct, not a machine-unreadable string

https://github.com/lima-vm/lima/blob/de0816ea4bdc5267b428ab21025889b8dd785526/pkg/driver/external/driver.proto#L207-L211

It should return a slice of struct, not a raw output of qemu-img snapshot -l which is not robustly parsable.

Depends on:

• https://github.com/lima-vm/go-qcow2reader/issues/75