allow non-unicode trimming element insertion path for transformer case

[MichaelBelousov]

#180 made it impossible for the transformer to preserve code values in the iModels that already have non-unicode spaces (e.g.non-breaking space,  ) persisted in a code value. We have to add an API for use by the transformer to preserve the old behavior during element insert, to prevent people forking an iModel, getting some code values from master using raw ECSQL (without wrapping them in a Code instance which would fix that) and trying to find them in a branch which had its code values "fixed" by this change.

Technically we could also say "use the Code class" but that sounds like it will lead to confusing bugs during ECSQL usage on some models. We can close this issue if the consensus is "use Code"