* Add generic arg to the `GatewayProxyFactoryBean`
Related to https://github.com/spring-projects/spring-integration/issues/3923
When gateway proxy is declared manually, a `GatewayProxyFactoryBean` is used as a `@Bean`.
In this case the info about target interface is not available on a `BeanDefinition`,
unlike with a programmatic registration via `MessagingGatewayRegistrar`.
* Expose `<T>` on a `GatewayProxyFactoryBean` to make end-user to specify the type
this gateway is going to be based on.
This allows Spring container to determine the type of the `FactoryBean` bean definition
properly
* Migrate a programmatic bean definition registration from the
`FactoryBean.OBJECT_TYPE_ATTRIBUTE` to the `targetType` property
of the `BeanDefinition` based on a `ResolvableType.forClassWithGenerics()`
to simulate generic arg for the application context
* Remove `ComponentsRegistration` from the `GatewayProxySpec` since it us out of use
* Fix affected tests for newly added generic arg on the `GatewayProxyFactoryBean`
* Fix another typo in gateway.adoc
Co-authored-by: Gary Russell <grussell@vmware.com>
Co-authored-by: Gary Russell <grussell@vmware.com>
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