Make it possible to make secrets optional

[marklonquist]

We like to standardize some of our secrets.yml file, eg always have an entry for the service.json secrets, but it is not always used. Can we make functionally to mark secrets as optional, so that secret-injector doesn't blow up? 😄

Example:

format: json
output: "/secrets"
# prefix: OLYMPUS_
secrets:
  - OLYMPUS/data/Namespaces/$K8S_CLUSTER_CLUSTER_NAMESPACE/Shared/$K8S_CLUSTER_OLYMPUS_ENVIRONMENT/misc:
      format: json
      filename: misc.json
  - OLYMPUS/data/Namespaces/$K8S_CLUSTER_CLUSTER_NAMESPACE/Services/$APP_NAME/$K8S_CLUSTER_OLYMPUS_ENVIRONMENT/service:
      format: json
      optional: true
      filename: service.json