Adds support for a config-sets.yml file at the root path where named bundles
of app configurations (env, hooks, secrets) can be defined and activated
per-instance or per-group, eliminating duplication across instance configs.

- Config-sets are defined as top-level keys in config-sets.yml
- Activate a set in any instance or group alaka.yml with CONFIG-SETS: <name>: true
- Disable a set inherited from a group with CONFIG-SETS: <name>: false
- Merge priority: group < config-set < instance
- Warns when a config-set is enabled but not defined in config-sets.yml
- Warns when a config-set targets an app not listed in the instance config
- Adds examples/config-sets.yml with bbb and calendar config-set examples

@@ -237,0 +280,4 @@
+        config_set_app_config = config_set_app_configs.get(app) or {}
+        instance_app_config = instance_apps[app] or {}
+        # Priority: group < config-set < instance
+        merged_config[app] = merge_dict(merge_dict(group_app_config, config_set_app_config), instance_app_config)

@@ -281,2 +323,4 @@
         dict: A dictionary with domains as keys and their respective merged configurations as values.
     """
+    if config_sets is None:
+        config_sets = {}