Add extraSecrets option to create additional Secrets

[maboehm]

Follow up to #93

Creating additional secrets allows users to safely pass confidential values to the deployment. When used in gardener, the ControllerDeployment would contain the literal secret values (which is ok, since they are protected/encrypted just like secrets).

So for example:

apiVersion: core.gardener.cloud/v1beta1
kind: ControllerDeployment
metadata:
  name: shoot-flux
type: helm
providerConfig:
  values:
    envFrom:
    - secretRef:
        name: super-secret
    extraSecrets:
    - name: super-secret
      stringData:
        foo: bar
    image:
      tag: latest