Add MQTT integration discovery (#41332)

* Add MQTT integration discovery

* Add script/hassfest/mqtt.py

* Unsubscribe if config entry exists

* Add homeassistant/generated/mqtt.py

* Fix bad loop

* Improve tests

* Improve tests

* Apply suggestions from code review

Co-authored-by: Fabian Affolter <mail@fabian-affolter.ch>

* Prevent initiating multiple config flows

Co-authored-by: Fabian Affolter <mail@fabian-affolter.ch>

script/hassfest/mqtt.py

@@ -0,0 +1,64 @@
+"""Generate MQTT file."""
+from collections import defaultdict
+import json
+from typing import Dict
+
+from .model import Config, Integration
+
+BASE = """
+\"\"\"Automatically generated by hassfest.
+
+To update, run python3 -m script.hassfest
+\"\"\"
+
+# fmt: off
+
+MQTT = {}
+""".strip()
+
+
+def generate_and_validate(integrations: Dict[str, Integration]):
+    """Validate and generate MQTT data."""
+
+    data = defaultdict(list)
+
+    for domain in sorted(integrations):
+        integration = integrations[domain]
+
+        if not integration.manifest:
+            continue
+
+        mqtt = integration.manifest.get("mqtt")
+
+        if not mqtt:
+            continue
+
+        for topic in mqtt:
+            data[domain].append(topic)
+
+    return BASE.format(json.dumps(data, indent=4))
+
+
+def validate(integrations: Dict[str, Integration], config: Config):
+    """Validate MQTT file."""
+    mqtt_path = config.root / "homeassistant/generated/mqtt.py"
+    config.cache["mqtt"] = content = generate_and_validate(integrations)
+
+    if config.specific_integrations:
+        return
+
+    with open(str(mqtt_path)) as fp:
+        if fp.read().strip() != content:
+            config.add_error(
+                "mqtt",
+                "File mqtt.py is not up to date. Run python3 -m script.hassfest",
+                fixable=True,
+            )
+        return
+
+
+def generate(integrations: Dict[str, Integration], config: Config):
+    """Generate MQTT file."""
+    mqtt_path = config.root / "homeassistant/generated/mqtt.py"
+    with open(str(mqtt_path), "w") as fp:
+        fp.write(f"{config.cache['mqtt']}\n")