Spring Integration Java DSL, is a tool to compose integration flows programmatically.
We can build the flow not only based on standard EIP components, but also using
protocol-specific channel adapters.
Any generic services also can be used as handler in the flow.
This includes simple lambda operations or functions.
On the other hand Spring Cloud Function provides a `FunctionCatalog`
for registered functions and their compositions & conversions.
With this change we introduce a more high-level DSL to use functions
from catalog directly in the `IntegrationFlow` to gain the best from
both worlds.
* Introduce `spring-cloud-function-integration` module based on
`spring-cloud-function-context` and `spring-boot-starter-integration`
* Expose a `FunctionFlowBuilder` auto-configuration
* Add `FunctionFlowDefinition` to expose `apply()` and `accept()` operators
* Document this new module
