[#19] feature: Create BestService

- BestService 생성

backend/src/main/java/com/team15/sidedish/service/BestService.java

@@ -0,0 +1,25 @@
+package com.team15.sidedish.service;
+
+import com.team15.sidedish.domain.Best;
+import com.team15.sidedish.domain.BestRepository;
+import org.springframework.stereotype.Service;
+
+import java.util.Set;
+
+@Service
+public class BestService {
+
+    private BestRepository bestRepository;
+
+    public BestService(BestRepository bestRepository) {
+        this.bestRepository = bestRepository;
+    }
+
+    public Set<Best> showAllBestDishes() {
+        return bestRepository.findAll();
+    }
+
+    public Best showSingleBestDish(Integer categoryId) {
+        return bestRepository.findById(categoryId).orElseThrow(IllegalArgumentException::new);
+    }
+}