Add OCI (Oracle Cloud Infrastructure) cloud provider support

[cvilodub]

Summary

Oracle Cloud Infrastructure (OCI) managed PostgreSQL, like AWS RDS and GCP
AlloyDB, does not grant the admin user full superuser privileges. This means
certain operations require the admin user to belong to the target
role(s) before they can succeed. Without a dedicated provider wrapper,
CreateDB and DropRole fail against OCI PostgreSQL.

This PR adds an OCI provider wrapper that mirrors the workarounds already in
place for AWS and GCP:

• CreateDB: grants the admin user membership in the target role before
  calling ALTER DATABASE ... OWNER TO, then delegates to the base
  implementation.
• DropRole: grants the admin user membership in both the role being
  dropped and the new owner role (required for REASSIGN OWNED BY), with
  appropriate handling for the "already a member" (0LP01) and "role does not
  exist" (42704) error codes. The new-owner grant is revoked via defer
  after the drop completes.

Changes

• pkg/config/config.go: add CloudProviderOCI constant and "oci" case in
  ParseCloudProvider.
• pkg/postgres/oci.go: new file implementing the ocipg wrapper with
  CreateDB and DropRole overrides.
• pkg/postgres/postgres.go: wire CloudProviderOCI into the NewPG factory.

Testing

Tested against an OCI PostgreSQL instance with the operator configured with
cloudProvider: OCI. Database and role lifecycle operations all complete successfully.