Add proper alter hooks for field rendering

Problem

The block field module doesn't quite utilise the BlockViewBuilder which is unfortunate because it means we miss some crucial Drupal hooks.

Solution

I tried to use the BlockViewBuilder but I wasn't able to reliably get the entity that should be rendered (I don't know if the block_field module actually creates it). For example if you use a view as a block in block_field then you can't find this in the viewElements method of the BlockFieldFormatter class.

Therefor I've just added the two alter hooks that are in the view builder in the viewElements method.

Work left

One of the things that still needs to happen is to use dependency injection in the field formatter. However, this was an existing problem so I suggest creating a follow-up issue to fix this.