Add ServerHttpResponse.addCookie method

This commit adds a `addCookie(ResponseCookie)` method to the reactive
`ServerHttpResponse` interface.

Issue: SPR-15523

spring-test/src/test/java/org/springframework/test/web/reactive/server/HttpHandlerConnectorTests.java

@@ -84,7 +84,7 @@ public class HttpHandlerConnectorTests {
 		TestHttpHandler handler = new TestHttpHandler(response -> {
 			response.setStatusCode(HttpStatus.OK);
 			response.getHeaders().put("custom-header", Arrays.asList("h0", "h1"));
-			response.getCookies().add(cookie.getName(), cookie);
+			response.addCookie(cookie);
 			return response.writeWith(Mono.just(toDataBuffer("Custom body")));
 		});