Rendering blocks does not pick up content format from the main request

Problem

The blocks sub-module is not picking up the main _content_format for rendering block content.

Steps to reproduce

Atm the code is hard-code to markup-rendering:

      foreach ($blocks as $block) {
        if ($render = $block->getPlugin()->build()) {
          $customElement = CustomElement::createFromRenderArray($render)
            ->toRenderArray();
          $output[$region][$block->id()] = $this->getrenderer()
            ->renderRoot($customElement);
        }

        if ($cacheableDependency) {
          $cacheableDependency->addCacheableDependency($block->getPlugin());
          $cacheableDependency->addCacheableDependency($block);
        }
      }

Proposed resolution

- read main request and apply desired content-format from there
- make use of \Drupal\custom_elements\CustomElementsBlockRenderHelperTrait::convertBlockRenderArray() in case no main custom element is provided in the render array.
- extend test coverage to ensure both markup and json rendering works