Use Direct Upload via Workhorse for CSV upload of user mapping

About

This issue is to add Direct Upload via Workhorse support for the CSV upload added in #455901 (closed). This will be a performance optimisation so the Rails process does not handle the file upload. From our docs:

Workhorse direct upload is a mechanism that lets us accept large uploads without spending a lot of Ruby CPU time.

Proposal

Following the recommendations in the working with uploads guide we should define a route for Workhorse Direct upload support.

Special merge request reviews

Have groupscalability review the MR, particularly for file-size considerations https://gitlab.com/gitlab-org/gitlab/-/issues/478305#note_2147212991.