PostgreSQL: model the new flavor field (upstream cozystack#4587) #63

cozystack/cozystack#4587 adds a top-level flavor enum to packages/apps/postgres/values.schema.json:

flavor: postgresql   # enum: postgresql | postgis, default postgresql

postgis bootstraps the cluster from the CloudNativePG PostGIS image. The provider does not model the field, so a cozystack_postgres resource cannot create a PostGIS cluster.

Two things to get right. The flavor is fixed at creation: the chart refuses to render a flavor change on an existing cluster, so an update surfaces as a failed HelmRelease rather than a silent image swap. Do not mark it RequiresReplace, which would turn a typo into a destroyed database; let the chart refuse the change. And do not restate the default with stringdefault.StaticString("postgresql"): a provider that sends postgresql on every plan would try to flip a PostGIS cluster created outside Terraform and hit that refusal on every apply. Model it Optional+Computed with UseStateForUnknown and a stringvalidator.OneOf("postgresql", "postgis"), and only send it when the user set it.