Add CharlestonMapper for object mapping (#7651)

This should resolve https://issues.dp.la/issues/7651

College of Charleston (one of three South Carolina Digital Library providers) unexpectedly moved to a new repository system and we had to change the OAI endpoint URL, the 'set' specification, and the mapping of the 'object' property as a result.

CharlestonMapper extends SCDLMapper in order to perform special mapping of the 'object' property that is different than that of the other SCDL providers.  The included test describes the desired behavior. 

Indiana is different because we actually lacked the 'handle' property that we manufacture from //oai_dc:dc/dc:identifier.

Having the insight I do now about 'handle' I could theoretically pull 'object' from 'handle' instead of '_id' if that would make something more clear; but I'm not sure it would, because of the mysterious nature of 'handle' in our code and the reader having to figure out where it comes from.