interceptor: use concrete types for clarity
interceptor: use concrete types for clarity

Rather than creating an interface that matches a small number (1 or 2)
API types, explicitly enumerate the API types.

google/trillian,AlCutter/trillian,RJPercival/trillian,daviddrysdale/trillian,gdbelvin/trillian,gdbelvin/trillian,Martin2112/trillian,google/trillian,Martin2112/trillian,RJPercival/trillian,AlCutter/trillian,daviddrysdale/trillian,Martin2112/trillian,daviddrysdale/trillian,gdbelvin/trillian