Extensibility: Override SCV mapping definitions

Users can now override Table Mappings to select a different source table or view for the mapping. Users can override Element Mappings to change the mapping definition for a target column, or to remove it.

All data transfers in SCV use mapping definitions to transfer data from a source table or view in a target table. Overriding mapping definitions allow users to change a data transfer in SCV without customizing the mapping definition and hence without customizing the macros running the corresponding data transfer.