POC - Routing based on HostHeader without tls-passthrough

[shaun-nx]

### Tasks
- [x] Diagram detailing deployment scenarios (mind map)
- [x] Detail security considerations
- [x] Create design document

[github-actions]

Hi @shaun-nx thanks for reporting!

Be sure to check out the docs and the Contributing Guidelines while you wait for a human to take a look at this 🙂

Cheers!