Support obtaining ServerWebExchange in ContextView (#6705)

#### What type of PR is this?

/kind feature
/area core
/milestone 2.20.x

#### What this PR does / why we need it:

This PR registers a ServerWebExchangeContextFilter to make ServerWebExchange available under ContextView. The usage example is as follows:

```java
Mono.deferContextual(contextView -> {
    var exchange = ServerWebExchangeContextFilter.getExchange(contextView);
    assertTrue(exchange.isPresent());
    return mono;
})
```

#### Does this PR introduce a user-facing change?

```release-note
None
```

application/src/main/java/run/halo/app/infra/config/WebFluxConfig.java

@@ -26,6 +26,7 @@ import org.springframework.http.codec.ServerCodecConfigurer;
 import org.springframework.http.codec.json.Jackson2JsonDecoder;
 import org.springframework.http.codec.json.Jackson2JsonEncoder;
 import org.springframework.lang.NonNull;
+import org.springframework.web.filter.reactive.ServerWebExchangeContextFilter;
 import org.springframework.web.reactive.config.ResourceHandlerRegistration;
 import org.springframework.web.reactive.config.ResourceHandlerRegistry;
 import org.springframework.web.reactive.config.WebFluxConfigurer;
@@ -252,4 +253,11 @@ public class WebFluxConfig implements WebFluxConfigurer {
         return new AdditionalWebFilterChainProxy(extensionGetter);
     }
 
+    @Bean
+    // We expect this filter to be executed before AdditionalWebFilterChainProxy
+    @Order(-102)
+    ServerWebExchangeContextFilter serverWebExchangeContextFilter() {
+        return new ServerWebExchangeContextFilter();
+    }
+
 }

application/src/test/java/run/halo/app/config/WebFluxConfigTest.java

@@ -1,6 +1,7 @@
 package run.halo.app.config;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.mockito.Mockito.when;
 
 import java.net.URI;
@@ -18,10 +19,15 @@ import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Import;
 import org.springframework.test.context.bean.override.mockito.MockitoSpyBean;
 import org.springframework.test.web.reactive.server.WebTestClient;
+import org.springframework.web.filter.reactive.ServerWebExchangeContextFilter;
+import org.springframework.web.reactive.function.server.RouterFunction;
+import org.springframework.web.reactive.function.server.RouterFunctions;
+import org.springframework.web.reactive.function.server.ServerResponse;
 import org.springframework.web.reactive.socket.WebSocketHandler;
 import org.springframework.web.reactive.socket.WebSocketMessage;
 import org.springframework.web.reactive.socket.client.ReactorNettyWebSocketClient;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 import run.halo.app.core.endpoint.WebSocketEndpoint;
 import run.halo.app.core.extension.Role;
@@ -31,7 +37,10 @@ import run.halo.app.extension.Metadata;
 
 @SpringBootTest(properties = "halo.console.location=classpath:/console/", webEnvironment =
     SpringBootTest.WebEnvironment.RANDOM_PORT)
-@Import(WebFluxConfigTest.WebSocketSupportTest.TestWebSocketConfiguration.class)
+@Import({
+    WebFluxConfigTest.WebSocketSupportTest.TestWebSocketConfiguration.class,
+    WebFluxConfigTest.ServerWebExchangeContextFilterTest.TestConfig.class
+})
 @AutoConfigureWebTestClient
 class WebFluxConfigTest {
 
@@ -154,4 +163,34 @@ class WebFluxConfigTest {
                 .expectStatus().isNotFound();
         }
     }
+
+
+    @Nested
+    class ServerWebExchangeContextFilterTest {
+
+        @TestConfiguration
+        static class TestConfig {
+
+            @Bean
+            RouterFunction<ServerResponse> assertServerWebExchangeRoute() {
+                return RouterFunctions.route()
+                    .GET("/assert-server-web-exchange",
+                        request -> Mono.deferContextual(contextView -> {
+                            var exchange = ServerWebExchangeContextFilter.getExchange(contextView);
+                            assertTrue(exchange.isPresent());
+                            return ServerResponse.ok().build();
+                        }))
+                    .build();
+            }
+
+        }
+
+        @Test
+        void shouldGetExchangeFromContextView() {
+            webClient.get().uri("/assert-server-web-exchange")
+                .exchange()
+                .expectStatus().isOk();
+        }
+
+    }
 }