Move root *.go files to gitlab-workhorse directory

What does this MR do and why?

This MR moves all *.go files from /workhorse root directory to a dedicated gitlab-workhorse folder, same as gitlab-resize-image, gitlab-zip-cat, and gitlab-zip-metadata: https://gitlab.com/gitlab-org/gitlab/-/tree/master/workhorse/cmd

How to set up and validate locally

The build process should work as always and create a gitlab-workhorse executable. If testing with GDK, then all HTTP requests should work correctly.

Related to #443564 (closed)