feat: add newsletter subscription

src/admin/admin.controller.spec.ts

src/admin/admin.controller.ts

src/admin/admin.module.ts

src/app.module.ts

...newsletter/interface/newsletter-subscription.interface.ts

src/newsletter/newsletter-subscription.schema.ts

src/newsletter/newsletter.controller.spec.ts

src/newsletter/newsletter.controller.ts

src/newsletter/newsletter.module.ts

src/newsletter/newsletter.service.ts