Allow theme blocks to be used

I need to be able to configure a block that is inserted between the rows of a view. Currently, this isn’t possible because we can only select a block plugin name.

Instead of adding support for the block form inside the ViewsStyle plugin, I have implemented a new row type called "Placed block", where block configuration entities can be selected.

With this approach, an admin can place a block in Block Layout, configure it, and render it between the view’s rows. The block can also be disabled in Block Layout, so it won’t be visible anywhere except inside the view.