Istio support in deployer

Deployer changes:

Adapted deployer to work with Istio.

Added istio field in clusterConstraints

Details in the schema doc.

x-google-marketplace:
  clusterConstraints:
    istio:
      type: OPTIONAL | REQUIRED | UNSUPPORTED

Added ISTIO_ENABLED property

Details in the schema doc.

• Deployer property

properties:
- name: istioEnabled
  type: boolean
  x-google-marketplace:
    type: ISTIO_ENABLED # Indicates whether the app is being deployed with Istio.

mpdev

Can now publish metadata to GCS using schema v2.