feat: support stream and chat_engine options for chats api

[Mini256]

For example

curl --location 'http://127.0.0.1:3000/api/v1/chats' \
  --header 'Content-Type: application/json' \
  --header 'Authorization: Bearer xxxxxx' \
  --data '{
      "messages": [
          {
              "role": "user",
              "content": "what is tidb?"
          }
      ],
      "index": "default",
      "chat_engine": "jira-bot",
      "stream": false
  }'

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