api: don't use unit struct (frb incompatibility)

[icota]

Skip parsing enum_or_struct `NamespacedName { namespace: Namespace { joined_path: "foundation_api::api::backup" }, name: "RestoreShardRequest" }` because of error (e=struct with unit fields are not supported yet, what about using `struct RestoreShardRequest {}` or `#[frb(opaque)] struct RestoreShardRequest;` instead)