Add hybrid approach

If an entities provider was registered use the default approach by calling sitemapUrlProvider->getSitemapUrl for every entity or object ELSE use sitemapUrlProvider->populate passing the new SitemapInterface
I think that we have to remove the concept of entitiesProvider.

SitemapUrl providers don't always need entities.

Of course, If you want to implement a sitemapUrl provider that populate the sitemap by loading enities, you can!!! 
Just injects what you want (a repository service for example), load your entities in the populate method, do your logic, populate the sitemap, i think is more flexible... i'm using this approach on a company project (humanitas) and it seems to works well...