chore(vault): require vaultRole, vaultMountPoint, and key

[silasbw]

No description provided.

[silasbw]

Long term we should consider adopting a per-backend type object. For example:

apiVersion: kubernetes-client.io/v1
kind: ExternalSecret
metadata:
  name: hello-service
spec:
  template:
    metadata:
      annotations:
        external-secret: 'Yes please!'
  vault:
    vaultRole: x
    vaultMountPoint: x
    key: x
  data:
    - key: hello-service/password
      name: password

Existing Kubernetes objects follow this pattern (e.g., volumes). It also makes for a more straightforward validation scheme that produces easy to understand error messages (the error message produced as a result of this PR are a little opaque).