Announcements structure and manager

**Is your feature request related to a problem? Please describe.**
Add Announcement class and cache enabled manager

Can be modeled after [GlobalCategoryManager](https://github.com/zaida04/guilded.js/blob/main/packages/guilded.js/lib/managers/global/CategoryManager.ts) [Category](https://github.com/zaida04/guilded.js/blob/main/packages/guilded.js/lib/structures/Category.ts)