Routing extensibility is limited by non-public API

This issue was transferred from a private repository

@mauroservienti built an Attribute Routing feature based on the routing API (presumed) extensibility. He had to use reflection to achieve the goal: https://github.com/mauroservienti/NServiceBus.AttributeRouting/blob/master/src/NServiceBus.AttributeRouting/AttributeRoutingFeature.cs#L19-L56

There's no way to understand which messages are scanned by the endpoint, or if a route has already been defined for a message.