Fixes https://github.com/spring-projects/spring-integration/issues/3047
* Improve `GatewayProxyFactoryBean` to determine the return type of the
method call from the interface generic types, when the `serviceInterface`
is a `java.util.function.Function`
* Propagate `MethodArgsHolder` as a `rootObject` for SpEL evaluations
* Deprecate `#gatewayMethod` and `#args` evaluation context variables
in favor of `MethodArgsHolder` as root object.
They will be removed in the future release and a single
`EvaluationContext` will be used for all the gateway expressions
* Introduce an
`IntegrationFlows.from(Class<?> serviceInterface, Consumer<GatewayProxySpec> endpointConfigurer)`
to allow to configure any valid gateway proxy options similar to what
we have with the `<gateway>` and `@MessagingGateway`.
This way we are very close to consistency between different approaches
* * Remove `default` prefix from `GatewayProxySpec` options
* Document the change
29bebdba97