Specify custom transformer input port for attributes in parameters

Currently, a custom transformer can have numerous Inputs, but when the user is setting a parameter value with attribute assignment permitted (or of the "Attribute Name" type), all attributes from all inputs are listed. This makes it difficult to use any type of dynamic merger/joiner inside the transformer, and significantly limits the user's ability to control how the transformer operates on different data. It can also lead to issues when both datasets have an attribute by the same name... or when they don't! It would be ideal if, instead of "Default" and "Off" for "Attribute Assignment," we could select a specific input port and only that input's exposed attributes would be listed.