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. 

@markbreedlove @no-reply :+1: from me with the one-liner assignment.