feat: 슬라이더 배너 API 구현

src/main/java/team18/team18_be/slider/controller/SliderController.java

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+package team18.team18_be.slider.controller;
+
+import io.swagger.v3.oas.annotations.tags.Tag;
+import java.util.List;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+import team18.team18_be.slider.dto.response.SliderImageResponse;
+import team18.team18_be.slider.service.SliderService;
+
+@Tag(name = "슬라이더", description = "슬라이더 관련 API")
+@RestController
+@RequestMapping("/api/slides")
+public class SliderController {
+
+  private final SliderService sliderService;
+
+  public SliderController(SliderService sliderService) {
+    this.sliderService = sliderService;
+  }
+
+  @GetMapping
+  public ResponseEntity<List<SliderImageResponse>> getSliderBanner() {
+    return ResponseEntity.ok().body(sliderService.getSliderImageUrls());
+  }
+}

src/main/java/team18/team18_be/slider/dto/response/SliderImageResponse.java

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+package team18.team18_be.slider.dto.response;
+
+public record SliderImageResponse(
+    String imageUrl
+) {
+
+}

src/main/java/team18/team18_be/slider/service/SliderService.java

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+package team18.team18_be.slider.service;
+
+import com.google.cloud.storage.Blob;
+import com.google.cloud.storage.Bucket;
+import com.google.cloud.storage.Storage;
+import java.util.ArrayList;
+import java.util.List;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.stereotype.Service;
+import team18.team18_be.slider.dto.response.SliderImageResponse;
+
+@Service
+public class SliderService {
+
+  public static final String IMAGE = "image/";
+  private static final String DIRECTORY_IMAGES = "Slider";
+  private final Storage storage;
+  @Value("${gcp.bucket.name}")
+  private String BUCKET_NAME;
+
+  public SliderService(Storage storage) {
+    this.storage = storage;
+  }
+
+  public List<SliderImageResponse> getSliderImageUrls() {
+    List<SliderImageResponse> imageUrls = new ArrayList<>();
+    Bucket bucket = storage.get(BUCKET_NAME);
+
+    for (Blob blob : bucket.list(Storage.BlobListOption.prefix(DIRECTORY_IMAGES)).iterateAll()) {
+      if (!blob.isDirectory() && blob.getContentType() != null && blob.getContentType().startsWith(
+          IMAGE)) {
+        String imageUrl = blob.getMediaLink();
+        imageUrls.add(new SliderImageResponse(imageUrl));
+      }
+    }
+
+    return imageUrls;
+  }
+}