Investigate "close" behavior of pluggable component instances

[philliphoff]

Runtime components can implement an optional Close() function called when the runtime is shutting down. This will serve as the basis for upcoming "hot reload" behavior in the runtime. It's not clear how this interacts with pluggable components, as none of the building block APIs have an explicit Close() function. While it's possible that the closure of the gRPC connection itself may serve as the signal for "closing" a component, it may not have quite the same meaning (e.g. an explicit closure by the runtime vs. a failure that results in the connection being closed). It might make more sense to have an explicit Close() function defined in the pluggable component APIs. (This would obviously be a breaking change, but given their current not-even-alpha state, would likely be acceptable.)

We need to spend some time investigating if and how to expose "close" behavior to pluggable component instances.