[component]: Header

[gfellerph]

Implement the header component, the one that brings all the other smaller parts together. This component should act as a container for the smaller parts, providing slots for putting things into as many of the smaller header parts are optional.

Design

https://www.figma.com/design/JIT5AdGYqv6bDRpfBPV8XR/Foundations-%26-Components-Next-Level?node-id=558-7012&t=saHtog1amMMrG60S-1

Requirements

• Provides slots for the global header, the local header and the main navigation
• Controls scrolling behaviour of the global header (stickyness)

Reference

### Tasks
- [ ] https://github.com/swisspost/design-system/issues/3635
- [ ] https://github.com/swisspost/design-system/issues/3636
- [ ] https://github.com/swisspost/design-system/issues/3626
- [ ] Create the `<post-header>` component
- [ ] Add e2e tests
- [ ] Add visual regression tests
- [ ] Design review