[cloud] [caching] Support @cached for subscriptions initial state load

In case of subscriptions that are on public data or don't have session variables in the authz rules for their models, the initial load can be massively sped up using `@cached`.

No file or test is named in the issue. Start by tracing the subscription initial-state load path and locating how the @cached directive is handled elsewhere. Define completion as cached subscriptions with public data or no session-variable authorization rules using the optimized initial load, with coverage for the affected cases.

Written by the indexing model from the issue text.