feat: created GroupUserDTO

src/main/java/no/ntnu/idi/stud/savingsapp/dto/goal/GroupUserDTO.java

+package no.ntnu.idi.stud.savingsapp.dto.goal;
+
+import lombok.Data;
+import lombok.NonNull;
+
+import java.math.BigDecimal;
+
+@Data
+public final class GroupUserDTO {
+
+  private Long userId;
+  private BigDecimal amount;
+}