Customizable Host headers

[brokad]

Currently the deployment router at the edge will use the value of the Host: header to decide where to route requests and by default will only accept my-project-name.shuttleapp.rs. This is not ideal for self-deployments which are not deployed as a subdomain of shuttleapp.rs.

We could expose a parameter in Shuttle.toml which lets users configure a Host key for their deployment.