Initial part of #232.
Extends the ActorTemplate API to add support for per-actor external
volumes and adds control plane and atelet hooks following the Actor
lifecycle.
Callouts to volume operations are abstracted with a Volume interface.
Right now, only a mock volume plugin for testing has been implemented,
but we will add CSI support next.
- [x] Tests pass
- [ ] Appropriate changes to documentation are included in the PR - Not
going to update documentation until we add CSI support.