Adding support for ActivateAttribute via IControllerActivator

This mechanism replaces code in Inject that activated properties
by looking them up by names.

First draft as a POC. Tests will come in later iterations.
Seems related to #617 - I like this change, but my understanding is that we wanted more discussion about the pattern in general since it's the anti-DI.

Did I miss that and there's a consensus now that this is OK?