Implement the pagination to RegisteredReactions

[dadamu]

Feature description

This issue is for the RegisteredReaction part's implementation of #448.
Each of registered reactions are associated to only one key so we can keep the current store logic.

Implementation proposal

Add the pagination field to RegisteredReaction, then revise it to have a paginated query.

[dadamu]

The current RegisteredReaction key is shortcode+subspace. In my opinion, it should be swapped to subspace+shortcode so that the user can query registered reactions in the subspace. Then we can also add new query method called QueryRegisteredReactionsWithSubspace.

What do you think?
@RiccardoM @bragaz

[RiccardoM]

@dadamu I agree with you on this change. Also, no need to add a new query. We can simply add a parameter to the QueryRegisteredReaction named SubspaceId. If SubspaceId is empty, we query for all reactions on all subspaces. Otherwise, we query only for those registered inside such subspace

[leobragaz]

@dadamu @RiccardoM I agree on both, we should consider to include subspaceID as part of a storeKey where's possible to make it easier to query elements based on their subspace