Add BlockHound exception for ConcurrentReferenceHashMap$ReferenceManager

This commit adds a SpringCoreBlockHoundIntegration configuration to
allows `pollForPurge` method to block.

Closes gh-33450

spring-core/src/main/java/org/springframework/core/ReactiveAdapterRegistry.java

@@ -442,10 +442,13 @@ public class ReactiveAdapterRegistry {
 		public void applyTo(BlockHound.Builder builder) {
 			// Avoid hard references potentially anywhere in spring-core (no need for structural dependency)
 
-			String className = "org.springframework.util.ConcurrentReferenceHashMap$Segment";
-			builder.allowBlockingCallsInside(className, "doTask");
-			builder.allowBlockingCallsInside(className, "clear");
-			builder.allowBlockingCallsInside(className, "restructure");
+			String segmentClassName = "org.springframework.util.ConcurrentReferenceHashMap$Segment";
+			builder.allowBlockingCallsInside(segmentClassName, "doTask");
+			builder.allowBlockingCallsInside(segmentClassName, "clear");
+			builder.allowBlockingCallsInside(segmentClassName, "restructure");
+
+			String referenceManagerClassName = "org.springframework.util.ConcurrentReferenceHashMap$ReferenceManager";
+			builder.allowBlockingCallsInside(referenceManagerClassName, "pollForPurge");
 		}
 	}