API endpoint for confirm order component for paid signup flow

This provides the API endpoint for the confirm order component for the paid signup flow

This MR is not functional on it's own, but instead is created in an effort to split up gitlab-org/growth/product#87 (closed) into smaller, more digestable MR's.

There is no changelog, since the feature is behind a feature flag.

Closes gitlab-org/growth/engineering#52 (closed), gitlab-org/growth/engineering#54 (closed), gitlab-org/growth/engineering#55 (closed)