Add workhorse internal upload API for observation only

Part of gitlab-com/gl-infra/scalability#1739 (closed)

This adds an internal API endpoint called by Workhorse when handling multipart MIME uploads on the catchall route. In this iteration we log the result of calling the API but we ignore the return value. We still rely on the old mechanism of deriving the temporary upload location from the document root.

This change will allow us to verify that the new API behaves as expected before we make it mandatory. This also has the benefit that it handles a gradual roll-out correctly.