Support standard GRANIAN_* env vars and the rloop event loop

- Launcher now only injects --interface/--host/--port when the matching
  GRANIAN_* env var is unset, so every granian option (GRANIAN_LOOP,
  GRANIAN_WORKERS, GRANIAN_HTTP, GRANIAN_HOST, GRANIAN_PORT, ...) can be
  configured via granian's standard GRANIAN_* environment variables
- Add the rloop event loop as a runtime dependency so GRANIAN_LOOP=rloop
  works out of the box (previously failed with a missing 'rloop' package)

requirements.txt

@@ -25,6 +25,8 @@ pwo==0.1.2
     # via
     #   kaya-core
     #   kaya-rsgi
+rloop==0.3.1
+    # via -r requirements.in
 typing-extensions==4.16.0
     # via
     #   kaya-core