* GH-3926: BeanNameGenerator for @MessagingGateway
Fixes https://github.com/spring-projects/spring-integration/issues/3926
Current approach for generated bean name in the `MessagingGatewayRegistrar`
is to decapitalize simple class name, which is similar to standard `AnnotationBeanNameGenerator`
* Make logic in the `MessagingGatewayRegistrar` based on the provided `BeanNameGenerator`
* Expose an `@IntegrationComponentScan.nameGenerator()` attribute to allow to customize
default bean name generation strategy
* Introduce `IntegrationConfigUtils.annotationBeanNameGenerator()` to
take a provided `AnnotationConfigUtils.CONFIGURATION_BEAN_NAME_GENERATOR` singleton
or fallback to the `AnnotationBeanNameGenerator.INSTANCE`
* Use this utility in the `IntegrationComponentScanRegistrar` if no custom strategy is provided
in the `@IntegrationComponentScan`
* Use same util from the `GatewayParser` since there is no custom naming strategy configuration
* Some other current Java level refactoring in the `IntegrationComponentScanRegistrar`
and `MessagingGatewayRegistrar`
* * Meta-annotate `@MessagingGateway` with a `@MessageEndpoint`
* Alias `@MessageEndpoint.value()` with a `@Component.value()`
* Alias `@MessagingGateway.name()` with a `@MessageEndpoint.value()`
* * Remove unused imports
* * Replace `MessagingGatewayRegistrar` parsing logic for annotation configuration
directly by the `GatewayProxyInstantiationPostProcessor` and `AnnotationGatewayProxyFactoryBean`.
* The `MessagingGatewayRegistrar` logic has been migrated back to the `GatewayParser`,
but only with an XML-relevant parts
* Change the logic of the `IntegrationComponentScanRegistrar` to rely on a `ClassPathBeanDefinitionScanner`
and its `scan()` functionality since this is all what we need to trigger a `GatewayProxyInstantiationPostProcessor`
for scanned components
* Fix `GatewayProxyInstantiationPostProcessor` to call an `afterPropertiesSet()` as well
* Add a `value()` alias attribute for the `@MessagingGateway` to satisfy a name resolution from a `@Component`
* Replace annotation chain resolution logic by new `MessagingAnnotationUtils.resolveMergedAttribute()` API
* Use `MergedAnnotations` API in the `AnnotationGatewayProxyFactoryBean` to preserve a logic for attribute
resolution from the annotation hierarchy
* Add expression resolution for attribute values in the `AnnotationGatewayProxyFactoryBean`
* Make a `GatewayInterfaceTests.CustomBeanNameGenerator` as an `AnnotationBeanNameGenerator`
extension to satisfy the test logic expectations: no custom name if explicit is present
* Clean up some doc typos after merge conflict
The fix in this commit essentially resolves some old JIRA ticket: https://jira.spring.io/browse/INT-4558
* * Remove redundant `MessagingAnnotationUtils.resolveMergedAttribute()`
* Optimize the logic in the `AnnotationGatewayProxyFactoryBean` around
annotation attributes to plain annotation - no adaptation to maps
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