Create external dependency module API/Interface(s)

We eventually want to support more dependency, monitored back-ends than just Consul and Vault. To enable this we need a standard API/Interface(s) that they could implement to work. This ticket is to track initial thoughts on what features each of these modules would need to implement to work as dependencies in hashicat.

Initial list of things that come to mind:
1. client
2. backend/client specific config
3. retry functions
4. views (they have consul specific fields)
5. ???

Start by reviewing the existing Consul and Vault dependency implementations and compare how they handle clients, backend-specific configuration, retries, and views. The work is complete when a documented standard interface covers the required dependency behavior and both existing backends can implement it.

Written by the indexing model from the issue text.