Feat: Better Reducer

[the-avid-engineer]

The idea of ICommand<TEntity> being a reducer was an opinionated abstraction, which is something I do want to avoid. To that end, I have removed the ICommand interface and added a Reduce method to the IEntity<TEntity> interface (which lives in the Common library, and can therefore be opinionated). All of the methods (other than the reducer) which took ICommand<TEntity> now take object

By default, this means that reduction is centralized to the entity for which the commands are relevant. However, consumers may still decentralize the reduction process by having their commands implement IReducer<TEntity> (new to the Abstractions library) and un-box the commands like this:

public TEntity Reduce(object[] commands)
{
    var newEntity = this;

    foreach (var command in commands)
    {
        if (command is not IReducer<TransactionEntity> reducer)
        {
            throw new NotImplementedException();
        }
        
        newEntity = reducer.Reduce(newEntity);
    }

    return newEntity;
}

The difference is that decentralization is no longer mandatory.