This adds the `Gitlab-Workhorse-Multipart-Fields` HTTP header, which
contains a list of signed multipart keys, for the CI artifacts upload
endpoints. This is already done for multipart attachments but was
not done for the the CI artifacts case.

Without this header, Rails can't guarantee that the file attachments
were validated by Workhorse.

This is the Workhorse part of the solution for
https://gitlab.com/gitlab-org/gitlab/-/issues/213139. This needs to be
used by Rails:
https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/403