introduce OGRLayer::FindFieldIndex() / OGR_L_FindFieldIndex() to lookup ...

In the normal usecase the NAS driver is  supposed to always update an preexisting schema.  For OCI there is a separate schema in the PostNAS project, that has it's identifiers truncated to a length that Oracle can cope with.  Unfortunately ogr2ogr currently doesn't seem to have a way to match and NAS attribute to its LAUNDERed counterpart in Oracle.

This PR introduces a new method OGRLayer::FindFieldIndex() that can be overloaded by the drivers to provide this kind of matching.   It also includes an update to ogr2ogr to make use of it and a implementation for the OCI driver, that tries to find the laundered version, if the full version doesn't exist.