Fetch functions require viewer as an argument, but Viewer is defined in keyserver - that's why we need to define this spec outside of lib.
Currently, we're accessing serverStateSyncSpecs props directly, but later in the stack this is going to be replaced by iterating over the specs.
https://linear.app/comm/issue/ENG-4631/migrate-checkstate-from-session-utils-to-a-spec
Depends on D8926