Move all settings resolving to the BlockContextManager

This PR moves all settings resolving to the BlockContextManager

- moved bundle settings from the ServiceLoader to the BlockContextManager, advantages are:
  - the block loader now only has the responsibility to load a block
  - the bundle settings are now available for the CMF (and others) that use a different block loader
- split-up bundle settings by type (service) and class, advantage is that there is no need to create a service if only the default settings are different
- introduced settings_by_class in the configuration, this allows to do: 

```
sonata_block:
    # ...
    blocks:
        # ...
        symfony_cmf.block.action:
            cache: symfony_cmf.block.cache.js_async
            settings_by_class:
                Symfony\Cmf\Bundle\BlockBundle\Document\RssBlock:
                    title: 'Insert the rss title'
                    url: false
                    maxItems: 10
                    template: 'SymfonyCmfBlockBundle:Block:block_rss.html.twig'
                    itemClass: 'Symfony\Cmf\Bundle\BlockBundle\Model\FeedItem'
```