Extensible var sources

Just as resource_types: extends the supported types under resources:, we should provide a way to extend the supported types under var_sources: so that credential managers and other types of var sources may be implemented externally.

Current proposal

Prerequisites:

• concourse/rfcs#37

Come up with an interface that prototypes can implement so that they can be used as a var source:

prototypes:
- name: vault-var-source
  type: registry-image
  source: {repository: concourse/vault-var-source-type}

var_sources:
- name: my-vault
  type: vault-var-source
  source: {...}

# ...

We must also ensure that the use of prototypes is secure and doesn't compromise the intent of using a credential manager in the first place.