Add ClientRequest attributes

This commit introduces client-side request attributes, similar to those
found on the server-side. The attributes can be used, for instance, for
passing on request-specific information to a globally registered
ExchangeFilterFunction.

The client request builder, as well as WebClient.RequestHeadersSpec and
WebTestClient.RequestHeaderSpec, add methods for adding a single
attribute, as well as manipulating the entire attributes map.

The client request itself adds a accessor for the (immutable) attributes
map.

This commit also introduces a new variant of the basic authentication
filter in ExchangeFilterFunctions. This variant takes the username and
password from well-known attributes.

Issue: SPR-15691

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultWebClientTests.java

@@ -145,6 +145,18 @@ public class DefaultWebClientTests {
 		client2.mutate().defaultCookies(cookies -> assertEquals(2, cookies.size()));
 	}
 
+	@Test
+	public void attributes() {
+		ExchangeFilterFunction filter = (request, next) -> {
+			assertEquals("bar", request.attributes().get("foo"));
+			return next.exchange(request);
+		};
+
+		WebClient client = builder().filter(filter).build();
+
+		client.get().uri("/path").attribute("foo", "bar").exchange();
+	}
+
 
 
 	private WebClient.Builder builder() {

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/ExchangeFilterFunctionsTests.java

@@ -25,7 +25,7 @@ import org.springframework.http.HttpHeaders;
 
 import static org.junit.Assert.*;
 import static org.mockito.Mockito.*;
-import static org.springframework.http.HttpMethod.*;
+import static org.springframework.http.HttpMethod.GET;
 
 /**
  * @author Arjen Poutsma
@@ -98,4 +98,23 @@ public class ExchangeFilterFunctionsTests {
 		assertEquals(response, result);
 	}
 
+	@Test
+	public void basicAuthenticationAttributes() throws Exception {
+		ClientRequest request = ClientRequest.method(GET, URI.create("http://example.com"))
+				.attribute(ExchangeFilterFunctions.USERNAME_ATTRIBUTE, "foo")
+				.attribute(ExchangeFilterFunctions.PASSWORD_ATTRIBUTE, "bar").build();
+		ClientResponse response = mock(ClientResponse.class);
+
+		ExchangeFunction exchange = r -> {
+			assertTrue(r.headers().containsKey(HttpHeaders.AUTHORIZATION));
+			assertTrue(r.headers().getFirst(HttpHeaders.AUTHORIZATION).startsWith("Basic "));
+			return Mono.just(response);
+		};
+
+		ExchangeFilterFunction auth = ExchangeFilterFunctions.basicAuthentication();
+		assertFalse(request.headers().containsKey(HttpHeaders.AUTHORIZATION));
+		ClientResponse result = auth.filter(request, exchange).block();
+		assertEquals(response, result);
+	}
+
 }