allow option to not install extensions

Please, answer some short questions which should help us to understand your problem / question better?

• Which image of the operator are you using? e.g. ghcr.io/zalando/postgres-operator:v1.13.0
  ghcr.io/zalando/postgres-operator:v1.14.0

• Where do you run it - cloud or metal? Kubernetes or OpenShift? [AWS K8s | GCP ... | Bare Metal K8s]
  bare metal

• Are you running Postgres Operator in production? [yes | no]
  yes

• Type of issue? [Bug report, question, feature request, etc.]
  feature request

Some general remarks when posting a bug report:

• Please, check the operator, pod (Patroni) and postgresql logs first. When copy-pasting many log lines please do it in a separate GitHub gist together with your Postgres CRD and configuration manifest.
• If you feel this issue might be more related to the Spilo docker image or Patroni, consider opening issues in the respective repos.

allow option to not install some subset of default extensions, such as pg_cron. if pg_cron is managed by us, it may be installed into a different database. the operator requires it to be installed in the postgres db.

No file or test is named in the issue. Start by locating the operator's default extension installation and configuration entry points, then trace how the postgres database requirement is enforced. Done means users can disable selected default extensions, including pg_cron, without breaking other extension handling.