[component]: Avatar

[gfellerph]

Create a component for user avatars.

Design: https://www.figma.com/file/xZ0IW0MJO0vnFicmrHiKaY/Components-Post?type=design&node-id=17162%3A7287&mode=design&t=Q7MUVh3G2mYSJ7OV-1

Props

• name
• company name (optional)

Slots

• bubble: slot for the <post-profile-picture> component (#3210)

### Tasks
- [ ] https://github.com/swisspost/design-system/issues/2596
- [ ] https://github.com/swisspost/design-system/issues/3210
- [x] Implement the new component `post-avatar`
- [x] Create a documentation page
- [x] Add cypress tests
- [x] Add visual regression tests
- [ ] Design review