Add opentherm_gw config flow (#27148)

* Add config flow support to opentherm_gw.
Bump pyotgw to 0.5b0 (required for connection testing)
Existing entries in configuration.yaml will be converted to config entries and ignored in future runs.

* Fix not connecting to Gateway on startup.
Pylint fixes.

* Add tests for config flow.
Remove non-essential options from config flow.
Restructure config entry data.

* Make sure gw_id is slugified

homeassistant/generated/config_flows.py

@@ -45,6 +45,7 @@ FLOWS = [
     "mqtt",
     "nest",
     "notion",
+    "opentherm_gw",
     "openuv",
     "owntracks",
     "plaato",