Introduce request attributes in RestClient

This commit introduces request attributes in the RestClient and
underlying infrastructure (i.e. HttpRequest).

Closes gh-32027

spring-web/src/test/java/org/springframework/http/client/InterceptingClientHttpRequestFactoryTests.java

@@ -115,6 +115,32 @@ class InterceptingClientHttpRequestFactoryTests {
 		request.execute();
 	}
 
+	@Test
+	void changeAttribute() throws Exception {
+		final String attrName = "Foo";
+		final String attrValue = "Bar";
+
+		ClientHttpRequestInterceptor interceptor = (request, body, execution) -> {
+			System.out.println("interceptor");
+			request.getAttributes().put(attrName, attrValue);
+			return execution.execute(request, body);
+		};
+
+		requestMock = new MockClientHttpRequest() {
+			@Override
+			protected ClientHttpResponse executeInternal() {
+				System.out.println("execute");
+				assertThat(getAttributes()).containsEntry(attrName, attrValue);
+				return responseMock;
+			}
+		};
+
+		requestFactory = new InterceptingClientHttpRequestFactory(requestFactoryMock, Collections.singletonList(interceptor));
+
+		ClientHttpRequest request = requestFactory.createRequest(URI.create("https://example.com"), HttpMethod.GET);
+		request.execute();
+	}
+
 	@Test
 	void changeURI() throws Exception {
 		final URI changedUri = URI.create("https://example.com/2");

spring-web/src/test/java/org/springframework/http/server/ServletServerHttpRequestTests.java

@@ -217,4 +217,10 @@ class ServletServerHttpRequestTests {
 		assertThat(request.getHeaders().getContentLength()).isEqualTo(result.length);
 	}
 
+	@Test
+	void attributes() {
+		request.getAttributes().put("foo", "bar");
+		assertThat(mockRequest.getAttribute("foo")).isEqualTo("bar");
+	}
+
 }

spring-web/src/test/java/org/springframework/web/util/ForwardedHeaderUtilsTests.java

@@ -17,6 +17,8 @@
 package org.springframework.web.util;
 
 import java.net.URI;
+import java.util.Collections;
+import java.util.Map;
 
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
@@ -363,6 +365,11 @@ class ForwardedHeaderUtilsTests {
 				return UriComponentsBuilder.fromUriString("/").build().toUri();
 			}
 
+			@Override
+			public Map<String, Object> getAttributes() {
+				return Collections.emptyMap();
+			}
+
 			@Override
 			public HttpHeaders getHeaders() {
 				return new HttpHeaders();