Autobridging Pt. 1

This is part of the autobridging code - it doesn't fully work yet, but I would like to merge it into develop. As currently configured, it will use the *original* OfferCreate transactor (the legacy transactor, `src/ripple_app/transactors/CreateOfferLegacy.cpp`) so nothing changes as far as transaction processing goes - at least it shouldn't.

Having it in develop makes it easier to maintain and will make the eventual merge easier since there won't be a ton of conflicts.