Add DataPolicy statements

Defines a named set of permissions that can be assigned to users or groups through collaborations. Policies contain PolicyStatements that specify allowed actions on different item types. When a user is granted a collaboration with a specific policy, they receive all permissions defined in that policy's statements. Common policies include viewer (read-only access), editor (read and write), and admin (full control). Policies are reusable and can be applied across many collaborations.