Service instances export a type attribute

Values can be "user_provided_service_instance" and
"managed_service_instance".

Ideally, the type attribute would just be the string class name. We'd
also use a presenter to hide (1) the arbitrary internal model namespace
and (2) the language-specific casing. But in the absence of said
presenter, we're just having the model handle the demodulizing and
underscoring for now.