feat: Add ASGI root path parameter to Phoenix server (#3186)

* Add ASGI root path parameter to Phoenix server

Allow for a root path parameter so that Phoenix can be hosted behind a proxy with a path prefix

* Name root path parameter more specifically

* Fix welcome message

* Enable client code to handle endpoints with path prefix

* Fix wrong endpoint in evaluation module

* Guarantee training slashes in host base URLs

* Remove host root path CLI parameter

CLI parameters for settings that are naturally adjusted via environment variables are discouraged for Phoenix

* Apply pre-commit fixes

* Fix and adjust unit tests