feat: created GroupRepository

src/main/java/no/ntnu/idi/stud/savingsapp/repository/GroupRepository.java

+package no.ntnu.idi.stud.savingsapp.repository;
+
+import no.ntnu.idi.stud.savingsapp.model.goal.Goal;
+import no.ntnu.idi.stud.savingsapp.model.goal.Group;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.stereotype.Repository;
+
+import java.util.Optional;
+
+@Repository
+public interface GroupRepository extends JpaRepository<Group, Long> {
+
+  Optional<Group> findBygGoals_Id(long id);
+}