[#55] 외부에서 주입한 설정 값을 사용할 수 있도록 수정

- FirebaseConfig의 ClassPathResource()를 PathResource()로 수정

src/main/java/me/liiot/snsserver/config/FirebaseConfig.java

@@ -7,6 +7,7 @@
 import org.springframework.beans.factory.annotation.Value;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.io.ClassPathResource;
+import org.springframework.core.io.PathResource;
 
 import javax.annotation.PostConstruct;
 import java.io.IOException;
@@ -29,7 +30,7 @@ public class FirebaseConfig {
     @PostConstruct
     public void initialize() {
         try {
-            InputStream serviceAccount = new ClassPathResource(apiKeyPath).getInputStream();
+            InputStream serviceAccount = new PathResource(apiKeyPath).getInputStream();
 
             FirebaseOptions options = new FirebaseOptions.Builder()
                     .setCredentials(GoogleCredentials.fromStream(serviceAccount))