postgresflex: support instance deletion protection #1800

Problem description

The Postgres Flex v3 API supports deletion protection for instances (POST .../instances/{instanceId}/protections, isDeletable in the instance response), but stackit_postgresflex_instance doesn't expose it yet.

Proposed solution

Add an optional deletion_protection attribute to the resource (and as computed to the data source). It gets applied via the protections endpoint after create/update.

resource "stackit_postgresflex_instance" "example" {
  # ...
  deletion_protection = true
}

Additional information

I'm already working on this and will open a PR..