Create external dependency module API/Interface(s)

[eikenb]

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. ???