Add principal relation entity

apps/api/src/app/database/entities/principal.entity.ts

+import {Entity, PrimaryColumn} from 'typeorm';
+
+@Entity()
+class Principal {
+  @PrimaryColumn()
+  public id: number;
+
+  @PrimaryColumn({
+    name: 'sub_id'
+  })
+  public subId: number;
+}
+
+export default Principal;