Introduce request attributes in RestClient

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

Closes gh-32027

spring-test/src/main/java/org/springframework/mock/http/client/MockClientHttpRequest.java

@@ -18,6 +18,8 @@ package org.springframework.mock.http.client;
 
 import java.io.IOException;
 import java.net.URI;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 import org.springframework.http.HttpMethod;
 import org.springframework.http.client.ClientHttpRequest;
@@ -46,6 +48,9 @@ public class MockClientHttpRequest extends MockHttpOutputMessage implements Clie
 
 	private boolean executed = false;
 
+	@Nullable
+	Map<String, Object> attributes;
+
 
 	/**
 	 * Create a {@code MockClientHttpRequest} with {@link HttpMethod#GET GET} as
@@ -115,6 +120,16 @@ public class MockClientHttpRequest extends MockHttpOutputMessage implements Clie
 		return this.executed;
 	}
 
+	@Override
+	public Map<String, Object> getAttributes() {
+		Map<String, Object> attributes = this.attributes;
+		if (attributes == null) {
+			attributes = new ConcurrentHashMap<>();
+			this.attributes = attributes;
+		}
+		return attributes;
+	}
+
 	/**
 	 * Set the {@link #isExecuted() executed} flag to {@code true} and return the
 	 * configured {@link #setResponse(ClientHttpResponse) response}.