Support more keys and values per secret

[knelasevero]

Related to #7. Also related to external-secrets/kubernetes-external-secrets#477 since we want to move in the new CRD direction, at least for this obvious good things.

Describe the solution you'd like
We would like to have multiple keys defined in the ExternalSecret, and end up with a kubernetes secret with multiple keys and values.

What is the added value?
We group multiple secrets that make sense to be together in one kubernetes secret.

Give us examples of the outcome
ExternalSecret:

apiVersion: secrets.externalsecret-operator.container-solutions.com/v1alpha1
kind: ExternalSecret
metadata:
  name: externalsecret-sample-2
spec:
  store_ref: 
    name: externalsecret-operator-secretstore-sample-2
    namespace: externalsecret-operator-system
  secrets:
    - key: operator-secret
      version: latest
    - key: operator-encrypted
      version: latest

Kubernetes Secret:

apiVersion: v1
kind: Secret
metadata:
  name:  externalsecret-sample-2
type: Opaque
data:
  operator-secret: <base64 value>
  operator-encrypted: <base64 value>

Constraints:

Limit secrets in ExternalSecret to reasonable number (to avoid misuse).