feat: CurrencyService outline & tests

src/main/java/eu/ztsh/wymiana/exception/InsufficientFundsException.java

@@ -0,0 +1,9 @@
+package eu.ztsh.wymiana.exception;
+
+public class InsufficientFundsException extends RuntimeException {
+
+    public InsufficientFundsException() {
+        super("Insufficient funds");
+    }
+
+}

src/main/java/eu/ztsh/wymiana/service/CurrencyService.java

@@ -0,0 +1,21 @@
+package eu.ztsh.wymiana.service;
+
+import eu.ztsh.wymiana.model.User;
+import eu.ztsh.wymiana.validation.InstanceValidator;
+import eu.ztsh.wymiana.web.model.CurrencyExchangeRequest;
+import lombok.RequiredArgsConstructor;
+import org.springframework.stereotype.Service;
+
+@RequiredArgsConstructor
+@Service
+public class CurrencyService {
+
+    private final UserService userService;
+    private final NbpService nbpService;
+    private final InstanceValidator validator;
+
+    public User exchange(CurrencyExchangeRequest request) {
+        throw new UnsupportedOperationException("Not implemented yet");
+    }
+
+}