GetAffected syntax in resource hook helpers

[maurei]

This PR adds some functionality to RelationshipDictionary<T> and DiffableEntityHashSet<T>: the GetAffected<T>(Func<T, object>) method. This method enables it to resolve the entities which have an updated value for the property as reflected in the navigation expression, similar to EF Core's Include( (a) => ( a.b)). This works for both relationships and members of the model itself.

Usage is like this:

public override IEnumerable<Article> BeforeUpdate(IDiffableEntityHashSet<Article> entities, ResourcePipeline pipeline)
{
    // gets the articles for which the `Author` relationship was updated.
    HashSet<Article> affectedByAuthor = entities.GetAffected( a => a.Author);

    // gets the articles for which the `Title` attribute was updated.
    HashSet<Article> affectedByTitle = entities.GetAffected( a => a.Title);

   // do some stuff with `affectedByAuthor` and/or `affectedByTitle`.

    return entities
}

This PR depends PR #538 to be merged first