Add type quick action for converting work items type

What does this MR do and why?

Related to #385227 (closed)

Adds the /type quick action to convert a work item to the specified type. This command is behind the feature flag work_items_mvc_2

Note that flash notifications are not yet implemented for the work item view so the command message will not be displayed (issue: Surface quick action errors (#391281)).

This MR also adds a new usage counter event (needed for QuickActions::InterpretService#usage_ping_tracking).

Screenshots or screen recordings

convert_work_item

How to set up and validate locally

1. In rails console enable the feature flag and create a task

   Feature.enable(:work_items_mvc_2)
   
   project, user  = Project.first, User.first
   type = ::WorkItems::Type.default_by_type(:task)
   task = WorkItem.create!(project: project, author: user, title: 'Test task to convert', issue_type: :task, work_item_type: work_item_type)
   task_url = "http://gdk.test:3000/#{project.full_path}/-/work_items/#{task.iid}"

2. Visit the task URL and try the quick action in a comment: /type Issue
3. Verify that a new note is created and the type changed to issue:

Click to expand screenshot

MR acceptance checklist

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