Typesafe endpoint return types

[daemontus]

Right now, all endpoints return OkResponse (or ErrResponse), which hold an arbitrary JSON string. Instead, it would be nice to have explicit types for some (all?) the endpoints, such that the objects can be automatically serialized through serde? For example, check_update_function could actually return Cardinality(f64) or ErrorMessage(String) instead of the generic response types.

This does not help that much on the JS side, since we have no type-checking there, but it helps future maintainers by clearly specifying what is/isn't a valid response from a particular endpoint.