Add checkNotModified support for view resolution

Issue: SPR-14522

spring-web/src/main/java/org/springframework/web/server/ServerWebExchange.java

@@ -68,6 +68,12 @@ public interface ServerWebExchange {
 	 */
 	Mono<WebSession> getSession();
 
+	/**
+	 * Returns {@code true} if the one of the {@code checkNotModified} methods
+	 * in this contract were used and they returned true.
+	 */
+	boolean isNotModified();
+
 	/**
 	 * An overloaded variant of {@link #checkNotModified(String, Instant)} with
 	 * a last-modified timestamp only.

spring-web/src/main/java/org/springframework/web/server/adapter/DefaultServerWebExchange.java

@@ -104,6 +104,11 @@ public class DefaultServerWebExchange implements ServerWebExchange {
 		return this.sessionMono;
 	}
 
+	@Override
+	public boolean isNotModified() {
+		return this.notModified;
+	}
+
 	@Override
 	public boolean checkNotModified(Instant lastModified) {
 		return checkNotModified(null, lastModified);