* GH-3118: MessagingGW: Don't proxy default methods
Fixes https://github.com/spring-projects/spring-integration/issues/3118
The `GatewayProxyFactoryBean` proxies all the methods in the provided interface,
including `default` and `static`.
This is not what is expected from end-users.
* Proxy only `abstract` methods from the provided interface
* Introduce a `DefaultMethodInvokingMethodInterceptor` to call `default`
method on the interface using a `MethodHandle` approach for those methods
calling
* * Introduce a `proxyDefaultMethods` option to let to
restore a previous behavior
* Handle a new property from XML, annotations & DSL configurations
* Ensure that the property works in various tests
* Document the feature
* * Fix typo in the `build.gradle`
c08f4ec5c6