[Feature Request] The stream of StoreResponse<T> should emit Loading(origin = SourceOfTruth)

Currently whenever we call Store.stream(StoreRequest.cached...)...) the only Loading we get is that of a Fetcher. This means that if we create a UI state folding the stream of StoreResponses, we can't really know when we are loading stuff from the SourceOfTruth, which is usually fast but not instant. Often we want to show a loading not only when we are fetching data but also while retrieving data from the source of truth.

The solution would be to add the emission of a Loading store response with the origin set to SourceOfTruth which retrieving data. This way we can be more granular the way we handle loading states.

Note: This refers to Store4 (4.0.5).

Start by tracing Store.stream(StoreRequest.cached(...)) and the SourceOfTruth loading path described in the issue. Confirm where StoreResponse.Loading emissions are created, then verify that source-of-truth retrieval emits Loading with origin SourceOfTruth without changing Fetcher loading behavior.