refactor(server): move host filtering to tower middleware (#1179)

* refactor: move host filtering to tower middleware

* fix tests

* commit missing file

* fix match on authority

* add some tests for authority

* grumbles: Arc<WhitelistedHosts>

* grumbles: split host filter code

* add example for host filter middleware

* Update examples/examples/host_filter_middleware.rs

* url.rs -> middleware/authority.rs