Feat: allows setting multiple types of default models in service config (#9404)

### What problem does this PR solve?

Allows set multiple types of default models in service config.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

conf/service_conf.yaml

@@ -64,9 +64,21 @@ redis:
 #   config:
 #     oss_table: 'opendal_storage'
 # user_default_llm:
-#   factory: 'Tongyi-Qianwen'
-#   api_key: 'sk-xxxxxxxxxxxxx'
-#   base_url: ''
+#   factory: 'BAAI'
+#   api_key: 'backup'
+#   base_url: 'backup_base_url'
+#   default_models:
+#     chat_model:
+#       name: 'qwen2.5-7b-instruct'
+#       factory: 'xxxx'
+#       api_key: 'xxxx'
+#       base_url: 'https://api.xx.com'
+#     embedding_model:
+#       name: 'bge-m3'
+#     rerank_model: 'bge-reranker-v2'
+#     asr_model:
+#       model: 'whisper-large-v3' # alias of name
+#     image2text_model: ''
 # oauth:
 #   oauth2:
 #     display_name: "OAuth2"