Provide `@WebSocketScope` annotation and public `SCOPE_WEBSOCKET` constant (for consistency with `@RequestScope` and co)

[hannah23280]

Spring framework : 6.2.8

Currently, to use the scope name for bean, we can use

BeanDefinition or ConfigurableBeanFactory interfaces which contain

1. SCOPE_SINGLETON
2. SCOPE_PROTOTYPE

WebApplicationContext interface which contain

1. SCOPE_REQUEST
2. SCOPE_SESSION
3. SCOPE_APPLICATION
   Alternatively, we can use @requestScope, @SessionScope and @ApplicationScope

For web socket scope, simply use the string "websocket" as the scope name

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Problem: The above usage is inconsistent. Why not, make it consistent across the board
Possible Suggestions:

1. BeanDefinition or ConfigurableBeanFactory interface contains all the 6 scopes

2. Introduce @SingletonScope, @PrototypeScope and @WebSocketScope

[jhoeller]

The basic BeanFactory arrangement only knows about singletons and prototypes, with special treatment for both of those and an extensible Scope SPI for additional scopes. Defining additional scope names at that level would be architecturally awkward since further scopes are only provided by higher-level arrangements such as WebApplicationContext, building on the extensible scope SPI just like any custom scopes would do. Those additional scopes need to be defined at the level that they are available on.

With respect to convenience annotations, @RequestScope, @SessionScope and @ApplicationScope mirror the corresponding Servlet attribute levels. All of those scopes are defined in proxy mode since they are primarily for bean references to be injected into common singleton beans, accessing the current request/session-scoped target bean on every method call. This is in strong difference to prototypes and in particular singletons which are almost always used without such scoped proxies.

Note that singletons are the default scope which make up >90% of all Spring beans, and prototypes are a rarely used pseudo scope: without full lifecycle management of the bean instances, rather just creating them and handing them off. As a consequence, potential @SingletonScope and @PrototypeScope annotations coming in proxy mode (consistent with the other annotations) would do more harm than good, not suggesting common usage at all.

Last but not least, point taken on the "websocket" scope which is a bit hidden in WebSocketMessageBrokerConfigurationSupport behind @EnableWebSocketMessageBroker. Making that scope name available as a public constant and providing a @WebSocketScope convenience annotation for it seems sensible since such an arrangement matches common usage. I'll narrow this issue for that purpose in 7.0.