Add support for headers in @HttpExchange

On the client side, supports `name=value` pairs. Placeholders in values
are resolved by the `embeddedValueResolver`.
On the server side, additionally supports `name` and `!name` syntax.

Closes gh-33309

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/RequestMappingHandlerMapping.java

@@ -449,7 +449,8 @@ public class RequestMappingHandlerMapping extends RequestMappingInfoHandlerMappi
 				.paths(resolveEmbeddedValuesInPatterns(toStringArray(httpExchange.value())))
 				.methods(toMethodArray(httpExchange.method()))
 				.consumes(toStringArray(httpExchange.contentType()))
-				.produces(httpExchange.accept());
+				.produces(httpExchange.accept())
+				.headers(httpExchange.headers());
 
 		if (customCondition != null) {
 			builder.customCondition(customCondition);