Adds Singlestatistic Option to Visualization Designer

What does this MR do and why?

Adds the Singlestatistic option to the Visualization Designer.

Screenshots or screen recordings

How to set up and validate locally

1. Make sure you are on GitLab Ultimate
2. You need to set up the Product Analytics DevKit if you haven't already done so. And follow the instructions on Connecting GDK to your devkit.
3. Access /-/product_analytics/dashboards/widget-designer on an product analytics activated project like [GDK_HOST]/gitlab-org/gitlab-shell/-/product_analytics/dashboards/widget-designer.

Setup for a project

1. Initialize the stack for a new project on the Rails console (rails c):

   project = Project.find_by_id(1)
   ProductAnalytics::InitializeStackService.new(container: project).execute
   # Expected response:
   # <ServiceResponse:0x00000001621a6db0 @http_status=:ok, @message="Product analytics initialization started", @payload={}, @reason=nil, @status=:success>

2. Go to /[namespace]/[project]/-/product_analytics/dashboards.

3. The setup page should render.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.