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

script/gen_requirements_all.py

@@ -138,6 +138,7 @@ TEST_REQUIREMENTS = (
     "pynws",
     "pynx584",
     "pyopenuv",
+    "pyotgw",
     "pyotp",
     "pyps4-homeassistant",
     "pyqwikswitch",