Allow sonata_page_render_block() to create blocks on-the-fly

I've worked on the `sonata_page_render_block()` Twig function to allow creating blocks on-the-fly.

This idea is related from issue https://github.com/sonata-project/SonataPageBundle/issues/264

For example, here is the Twig code for creating a `sonata.block.service.text.block` block on-the-fly in the `content_top` container:

```twig
{{ sonata_page_render_block('my.template.text.block', {
    type: 'sonata.block.service.text',
    container: 'content_top',
    settings: { content: 'Hi! this is my text content' }
}) }}
```

Block is created once and so not created again on the next parse if it already exists.