Resolves https://github.com/spring-cloud/spring-cloud-stream/issues/2453
Previously, to configure the use of an alternative exchange (used to route
messages when no queue is bound), the user had to manually declare the
exchange and any bindings, and modify the destination exchange using a
`DeclarablesCustomizer` bean.
Add first class support to configure the destination exchange and, optionally,
provision the alternate exchange as well as optionally binding a specific
queue to it.
Resolves#2502
4fda96507c