* GH-3533: Register WebSocket endpoints at runtime
Fixes https://github.com/spring-projects/spring-integration/issues/3533
* Rework `WebSocketIntegrationConfigurationInitializer` to register beans functional way
to avoid reflection for Spring Native support
* Move `IntegrationServletWebSocketHandlerRegistry` into a separate file for better readability
* Implement `DestructionAwareBeanPostProcessor` for `IntegrationServletWebSocketHandlerRegistry`
to track runtime bean registrations and removals
* Introduce an `IntegrationDynamicWebSocketHandlerMapping` to manage runtime mapping
registrations and removals
* Add `servlet-api` dependency into `websocket` to be able to compile an
`IntegrationDynamicWebSocketHandlerMapping`
* Fix typo in the exception message of the `StandardIntegrationFlowRegistration`
* Start dynamically added beans together with associated `IntegrationFlow` in the
`StandardIntegrationFlowContext`
* Document new feature
* * Fix language in docs
* Don't start those `SmartLifecycle`s together with a dynamic flow
which are not `isAutoStartup()`
* * Fix `TomcatWebSocketTestServer` to configure servlet for `loadOnStartup = 1`
* Fix `WebSocketDslTests` to make `clientWebSocketContainer.setAutoStartup(true)`
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