APIs for simulated payments (bug 1050593)

The only client using it is Spartacus; this endpoint needed to return data which is why it changed to a 200: https://github.com/mozilla/spartacus/commit/1bbc9dbdda3e9384505201007049b91a6569f6ba#diff-ac7c24ec9cb69d20c00a4b7173eb21d5R132