Add aggregate configuration support and update documentation

- Introduced AggregateConfig to manage aggregation settings in the gateway configuration.
- Added validation for reserved alias 'agg' and included tests for aggregate alias handling.
- Updated config.example.yaml to demonstrate aggregate configuration options.
- Enhanced README.md to include information about the new aggregation endpoint and its usage.
- Modified gateway.go to integrate the new aggregate handler for processing aggregation requests.

config.example.yaml

@@ -23,6 +23,12 @@ whitelist_cidrs:
 #   - "10.0.0.0/8"
 trusted_proxies: []
 
+# Опционально: по умолчанию /api/agg/* опрашивает все servers; можно ограничить список:
+# aggregate:
+#   include_aliases:
+#     - gt1
+#     - gt2
+
 servers:
   - alias: main_srv
     base_url: http://127.0.0.1:9091