feat : add unattachedSince date to users without any structure

scripts/init-db.js

src/admin/admin.controller.ts

src/migrations/scripts/1666104879255-add-unattachedSince.ts

src/structures/services/structures.service.ts

src/structures/structures.controller.spec.ts

src/structures/structures.controller.ts

src/users/dto/create-user.dto.ts

src/users/schemas/user.schema.ts

src/users/services/users.service.spec.ts

src/users/services/users.service.ts