Non-template based dependency watching

Create a non-template notifier and supporting code to allow monitoring arbitrary dependencies without the need for a template.

Use cases in CTS and Envconsul as well as internal vault token watcher.

Not necessary but something to keep an eye on... could template dependencies reuse this system as a lower level abstraction?

No files or tests are named. Start by tracing the existing template-based dependency watching and reviewing the CTS, Envconsul, and Vault token watcher use cases. Done means arbitrary dependencies can be monitored through a non-template notifier, with the relationship to template dependencies decided and covered by appropriate tests.

Written by the indexing model from the issue text.