[Feature] Extend Channel Type Routing Support

[iancooper]

Is your feature request related to a problem? Please describe.

We use a datatype channel approach to routing, assuming that the type of the channel is the type we need to find a mapper for, and then route to a handler.

Other topologies can exist though. Header-based routing would indicate the type in a header value - perhaps following Cloud Events - and Content based routing would use the message body. Both of these latter types allow mixed message types on the queue/stream.

Describe the solution you'd like

• The subscription needs a property which takes an enumerated value RoutingType that defaults to DataType but can include Header or Content
• When we look up the mapper for an incoming message we should do so according to this rule - so DataType routing channels are configured with their type, but other channels may need a delegate that we call back to pick up the type from
  Header or Content. We may provide an off-the-shelf delegate for Cloud Events
• Once we have the type, routing via CP is as normal

Describe alternatives you've considered

The alternative is just to use Cloud Events everywhere and just populate with the channel type for datatype channels on a post

Additional context

This is a stretch goal for V10, possibly during RC 1.