Add ports and service for product affinity functionality

src/main/java/com/example/productaffinity/domain/port/incomming/GetAffinityProductsUC.java

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+package com.example.productaffinity.domain.port.incomming;
+
+import com.example.productaffinity.domain.model.Product;
+import reactor.core.publisher.Mono;
+
+import java.util.List;
+
+// Input Port for resolving similar products to a requested product
+public interface GetAffinityProductsUC {
+    Mono<List<Product>> getAffinityProducts(String productId);
+}

src/main/java/com/example/productaffinity/domain/port/outcoming/AffinityProductsIdsPort.java

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+package com.example.productaffinity.domain.port.outcoming;
+
+public class AffinityProductsIdsPort {
+}

src/main/java/com/example/productaffinity/domain/port/outcoming/ProductDetailsPort.java

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+package com.example.productaffinity.domain.port.outcoming;
+
+import reactor.core.publisher.Mono;
+
+import java.util.Collection;
+
+public interface ProductDetailsPort {
+    Mono<Collection<String>> findAffinityProductIds(String productId);
+}

src/main/java/com/example/productaffinity/domain/service/SimilarProductsService.java

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+package com.example.productaffinity.domain.service;
+
+import com.example.productaffinity.domain.model.Product;
+import com.example.productaffinity.domain.port.incomming.GetAffinityProductsUC;
+import org.springframework.stereotype.Service;
+import reactor.core.publisher.Mono;
+
+import java.util.List;
+
+@Service
+public class SimilarProductsService implements GetAffinityProductsUC {
+
+    public SimilarProductsService() {
+    }
+
+    @Override
+    public Mono<List<Product>> getAffinityProducts(String productId) {
+        return null;
+    }
+}