* GH-3374: Fix scan for BF propagation
Fixes https://github.com/spring-projects/spring-integration/issues/3374
An internal `ClassPathScanningCandidateComponentProvider` instance in the `IntegrationComponentScanRegistrar`
does not propagate a provided `registry`.
* Implement `getRegistry()` on the internal `ClassPathScanningCandidateComponentProvider` to propagate
a provided into the `registerBeanDefinitions()` a `BeanDefinitionRegistry`
* Add `@Conditional` on some scanned `@MessagingGateway` in the `EnableIntegrationTests`
**Cherry-pick to 5.3.x & 5.2.x**
* * Remove unused import
* Restore `unused` warning on the unused registry arg
5e98f6d7f3