Add schema markup to breadcrumb

What does this MR do?

This MR adds schema markup for breadcrumbs. It adds it using ld+json because the breadcrumb generation logic is so intricate that it's really messy to introduce the microdata tags.

Refs #36844 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Related to #36844 (closed)

app/helpers/breadcrumbs_helper.rb

@@ -32,4 +32,46 @@ def add_to_breadcrumb_dropdown(link, location: :before)
     @breadcrumb_dropdown_links[location] ||= []
     @breadcrumb_dropdown_links[location] << link
   end
+
+  def push_to_schema_breadcrumb(text, link)
+    list_item = schema_list_item(text, link, schema_breadcrumb_list.size + 1)
+
+    schema_breadcrumb_list.push(list_item)
+  end
+
+  def schema_breadcrumb_json
+    {
+      '@context': 'https://schema.org',
+      '@type': 'BreadcrumbList',
+      'itemListElement': build_item_list_elements
+    }.to_json
+  end
+
+  private
+
+  def schema_breadcrumb_list
+    @schema_breadcrumb_list ||= []
+  end
+
+  def build_item_list_elements
+    return @schema_breadcrumb_list unless @breadcrumbs_extra_links&.any?
+
+    last_element = schema_breadcrumb_list.pop
+
+    @breadcrumbs_extra_links.each do |el|
+      push_to_schema_breadcrumb(el[:text], el[:link])
+    end
+
+    last_element['position'] = schema_breadcrumb_list.last['position'] + 1
+    schema_breadcrumb_list.push(last_element)
+  end
+
+  def schema_list_item(text, link, position)
+    {
+      '@type' => 'ListItem',
+      'position' => position,
+      'name' => text,
+      'item' => link
+    }
+  end
 end