Automatically create operator-level identity in Kratos on startup

[njlie]

When a multi-tenant instance of Rafiki starts up, it should automatically create a user role that represents the operator of the Rafiki instance. We'll call this role operator, and represent it in the Kratos Identity object as an admin-only metadata field where the field name is the name of that role, and its value should be true. The backend should not accept an identity as an operator unless the value of its field in the metadata is true.

[njlie]

Closed in #2909.