Enable Operation._add_control_inputs() with the C API and related improvements

This change:
 - Implements the C API logic for Operation._add_control_inputs()
 - Adds type-checking to Operation._add_control_input()
 - Makes Graph::AddControlEdge() update the node def if necessary
 - Makes Graph::AddControlEdge() a no-op if the control edge already exists

The AddControlEdge() changes may have a performance impact if anything
is sensitive to AddControlEdge(), but nothing is to my knowledge. I'm
not sure what benchmarks would confirm this.

PiperOrigin-RevId: 172158589