feat: created group model.

src/main/java/no/ntnu/idi/stud/savingsapp/model/goal/Group.java

+package no.ntnu.idi.stud.savingsapp.model.goal;
+
+import jakarta.persistence.*;
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+import lombok.NonNull;
+import no.ntnu.idi.stud.savingsapp.model.user.User;
+
+import java.math.BigDecimal;
+import java.util.List;
+
+@Data
+@AllArgsConstructor
+@NoArgsConstructor
+@Entity
+@Table(name = "group")
+public class Group {
+
+  @Id
+  @GeneratedValue(strategy = GenerationType.IDENTITY)
+  @Column(name = "group_id")
+  private Long id;
+
+  @OneToMany
+  @Column(name = "goals")
+  private List<Goal> goals;
+
+  @ManyToOne
+  @JoinColumn(name = "user_id")
+  private User creator;
+}