Add single_config_entry manifest option (#109505)

* Allow setting if we support multiple config entries in config flow

* Move property to config flow instead of flow handler

* Move marking an integration as single instance only to manifest

* Revert line remove

* Avoid init a config flow or adding a new entry on a single instance with an entry

* Revert changes in test

* Process code review comments

* Apply code review suggestion

script/hassfest/config_flow.py

@@ -191,6 +191,11 @@ def _generate_integrations(
                 if integration.iot_class:
                     metadata["iot_class"] = integration.iot_class
 
+                if single_config_entry := integration.manifest.get(
+                    "single_config_entry"
+                ):
+                    metadata["single_config_entry"] = single_config_entry
+
             if integration.integration_type == "helper":
                 result["helper"][domain] = metadata
             else: