Create regression testing items in batch

Attempts to create each regression testing item in the request and continues when individual items fail. The response includes both successfully created items and per-item errors. For apps using FlightPaths with multiple destinations, include stepConfigId on each item when the target regression step would otherwise be ambiguous. One-off items (oneOffForVersion set) on FlightPaths apps can be created with an initial status, which avoids a separate status update per item; items whose status is invalid are reported in errors and the rest are still created.