feat(testing): support reusable request customizers

flash-testing/src/main/java/dev/relism/flash/testing/FlashRequest.java

@@ -7,6 +7,7 @@ import java.net.http.HttpResponse;
 import java.nio.charset.StandardCharsets;
 import java.time.Duration;
 import java.util.Objects;
+import java.util.function.Consumer;
 
 /**
  * A request being built against a {@link FlashTest} server. The HTTP verb is terminal — it
@@ -41,6 +42,12 @@ public final class FlashRequest {
         return this;
     }
 
+    /** Applies {@code customizer} — how an extension's test kit attaches a credential, for instance. */
+    public FlashRequest with(Consumer<FlashRequest> customizer) {
+        customizer.accept(this);
+        return this;
+    }
+
     /** Sets a UTF-8 request body. */
     public FlashRequest body(String text) {
         this.body = Objects.requireNonNull(text, "text").getBytes(StandardCharsets.UTF_8);